What do the 2 different sizes of reel seats mean? Are the specific to certain types of reels/blanks or is it more of a personnel thing and just varies on which type you like?

16mm or 17mm is the diameter. All reels will fit either.   It's personal preference for the most part but you do want to match up with blanks and grips.  I build using both.  You don't really want to put a 17mm seat on an ulra light and if you are building on a mag bass blank then a 16 makes your handles tricky sometimes.

16mm or 17mm is the diameter. All reels will fit either. It's personal preference for the most part but you do want to match up with blanks and grips. I build using both. You don't really want to put a 17mm seat on an ulra light and if you are building on a mag bass blank then a 16 makes your handles tricky sometimes.

Alright thank you

There is a little more to it than preference.  Ergonomically, the 17 or 18 are better for most hand sizes, but each adds about half an ounce of mass relative to the next smaller size.  I use 17's on all freshwater spin rods exc the ultralights.  About 8 hours in, you'll appreciate the difference.  For all but the ultralights, unless you're building a really heavy power rod, I recommend the 17.

Use the size that feels best in your hands, with the reel mounted.