Third option in PRV. Select all notes. Grab the right end of one note. Drag it to the right. All notes will expand same number of "pixels".
Beware that if you inserted some longer notes they will get longer by the same number of pixels as the smaller ones but will not be the same size as the resultant smaller ones.
HTH.
Ps. I did write a CAL script years ago that set all notes to the same length (that you provide). PM me if you want that CAL script (assuming you know how to install and run CAL scripts).