I have, and love, MFT stamp sets. I have used a 3x3 or 4x4 block for the larger images like the lady in the chair from Here's the Scoop. Other than that, I always use a 2x3 for long sentiments and other images. I got most of my blocks from
Ammiki Enterprises, an Ebay seller. I have a few blocks from CTMH but that was when I first started buying acrylic stamps and didn't know any different. I hope this helps!