What size SSD drive is adequate?

I have a 500GB Samsung EVO 850 SSD and a 2TB hard drive.  I have Win 10 and programs (both DAW and regular PC programs) on the SSD (339GB used) but I have my Sonar projects on the 2TB hard drive.  All my "completed" Sonar sessions (mixed down masters) are also on the 2TB hard drive.
 
Many new PC's (commercially built such as Dell, HP, etc) now come with an M.2 SSD (generally 128 or 256 GB) and a hard drive.  Most programs, data storage, etc is on the hard drive.  

I think this depends a lot of what you plan to do. If you plan to buy large sample libraries then I would get the largest SSD's you can. Notice I have 3 of them in my sig and I'm going top need another one. Sample libraries being the main culprit.
At one time it was a common suggestion to buy a smaller HDD for the OS and use SSD for the samples and storage. The reason being retrieval times are not as necessary on the OS drive and more important when pulling from a sample library. While this works ok you are still buying a mechanical drive more prone to failure if you buy an HDD for the OS drive. For this reason I like to keep everything in the SSD realm.
 
If only loading  medium sized daw mix downs with few samples as whole projects, any outboard storage drive will probably be ok so long as the project can be pulled onto a faster drive later. If loading lots of synths that want to reside on C drive, then a larger C drive is a must.
 
Many MOBOs have accommodation for multiple drives. At least four or more. Since the  500g SSD drives cost less than the larger SSD drives, it makes sense to use many smaller drives as opposed to one huge drive. In this way if you experience a failure on one drive you won't loose everything. I think it also makes sense to store very important things you value on a drive that powers down and in this way reduces the time the drive is online thereby reducing failure rates and the possibility of loss. Of course you can't do that with a sample drive you regularly use. In my case it's not difficult to simply add another drive. Buy it, plug it in and away you go. I keep one drive as primarily a storage drive and the other as a sample drive....but as others have already mentioned, lots of programs default to C drive folders. You should be able to re direct the folders and point the program to them. The C drive tends to get filled up with junk over time anyway and you need to be careful when cleaning house because you could delete something important. This is why I like to err on too much space instead of too little.
 

If you keep the boot drive as clean/lean as possible (which also helps for backups), you can easily get by with a 250GB SSD.  You can fit a lot of programs/plugins in 250GB space.  (Not talking about large accompanying sample-libraries - put those on a dedicated drive)
 
FWIW, I'd want a 3rd drive dedicated to mixed-down Wav and MP3 files... so they're not taking space away from your "Audio" drive.  Note that conventional HDs start to slow down as they reach the ~2/3 full point.

For displaying your current disk and folder usage, I have found the free WinDirStat indispensable.
 
https://windirstat.net/
 

WinDirStat is a free and open-source graphical disk usage analyzer for Microsoft Windows. It is notable for presenting a sub-tree view with disk use percentage alongside a usage-sorted list of file extensions that is interactively integrated with a colorful graphical display.

 
I would recommend putting Windows, all programs (except content), and plugins (except samples), on your main system drive, using an SSD at least 250GB.  I keep mine at around 50% used this way.
 
This provides for very snappy performance of boot, program launch, project opening, etc., without any lag. 
 
HDDs are fine for audio files, projects, content, samples, etc.  The use of a 2nd or 3rd drive is up to personal needs and preferences. 
 
The use of any available program and plugin installer options to locate content and sample libraries on a secondary drive at install time is the best way to keep this stuff off of your primary SSD drive.  For example, Native Access and the XLN Online Installer have options to set up the paths for you.  Cakewalk has content path options as well.
 
Then the use of directory junctions is the recommended route for installers that won't cooperate with you and insist on dumping everything on the C: drive.  Directory junctions will let your application think the data is still on the C: drive, but the directory junction tricks it with a type of shortcut that points to the actual location. 

Those are both excellent deals.  If it were me (and it would be me if my system weren't 7 years old and I wasn't planning on buying a new one next year) I'd get the 500Gb one.  I'd even consider picking up the 1TB one for $189 for samples if I used a lot of them.

Shoot, I've been just fine with a 160GB SSD for my OS/program drive.

The system drive in my current DAW is a 256GB SSD, which is more than adequate. My previous DAW (which is now the family computer) has a system drive of a mere 111GB, and there is still plenty of free space on it. I'm running Windows 7 though. I'm guessing that Windows 10 has more bloat.

Price per GB in SSD SATA-III drives is cheapest in the 500-512GB range, so to answer the OP's question, I would go with 512GB at a minimum.
 
Furthermore, it appears the Samsung 860 EVO is one of the best, fastest SATA drives right now carrying a relatively low price.

FWIW, I just got back from Best Buy with a new 860 EVO 500GB.  $119...$2 more than Amazon. Helping keep the local businesses alive, lol!

So, I ended up going with two 2tb standard HHDs and, will probably add a 3rd one soon. I like plenty of storage.