Do you guys change your buffer size after you’ve finished tracking? In other words, do you increase the buffer size when you start mixing?

I don't. I monitor through my interface as I'm tracking, so latency isn't an issue for me.

I do alter the buffer depending on what I'm doing.

I often do, depending on what's in the project.

I keep my buffers at the maximum all the time. With my current interface, that's 2048. My previous interface could go to 16384, which was excessive so I never put it higher than 4096.
 
I get away with this because latency is not a concern. My process was developed long ago when computers just weren't powerful enough to bring latency down low enough, and still serves me well.
 
I monitor live audio input through the interface, so there's no appreciable latency.
 
I do not play soft synths in real time. I do, however, use a lot of soft synths and samplers and always play every part in real time on a keyboard. I manage that by using an outboard synthesizer that I can hear in real time. After recording the performance, I'll then re-route the recorded MIDI to a soft synth or sample library.

bitflipper
I keep my buffers at the maximum all the time. With my current interface, that's 2048. My previous interface could go to 16384, which was excessive so I never put it higher than 4096.
 
I get away with this because latency is not a concern. My process was developed long ago when computers just weren't powerful enough to bring latency down low enough, and still serves me well.
 
I monitor live audio input through the interface, so there's no appreciable latency.
 
I do not play soft synths in real time. I do, however, use a lot of soft synths and samplers and always play every part in real time on a keyboard. I manage that by using an outboard synthesizer that I can hear in real time. After recording the performance, I'll then re-route the recorded MIDI to a soft synth or sample library.

Wow, I do the exact opposite! My buffers are down as far as they go, which is around 45 I think (the 1st gen Focusrite settings have really odd settings).
 
Only in rare cases do I need to up it due to latency.
 

Yes,  I track at 256 which is as low as my computer can tolerate and then bump up to 1024 for mixing.

My buffers are down to 128 or so..
I don't change anything when i go to mix. 
Haven't had to in a while.