I used to keep 1.5" squares for a postage stamp quilt. However, due to health issues, I've abandoned this goal so no longer save them.
I currently save nothing smaller than 2 inch square and 1.5 x 4 inch strips.
I put my leftovers into a scrap box. After a year (normally), I cut everything up into predetermined sizes. I preplan what my scrap quilts will be in order to decide what sizes to save.