From bassboy1 in thread; https://www.tinboats.net/forum/viewtopic.php?f=13&t=6588&start=10
... For size, there are a lot of options. For my structure, it is pretty much all 3/16 rivets. These things are very strong. I used a combination of 3/16, 5/32 and 1/8 inch to hold my deck and side paneling down. Here is my recommendation. The deck holds just fine with 1/8 inch rivets, and they won't be felt through carpet. Plus, they are cheap - twice as many in the same price box as the other two sizes. I put them at 6 inch intervals, which is probably overkill, but that is the way I roll.
On side paneling, I would recommend using 1/8 on things that are there as side panels. On panels that also serve as a gusset (on a lot of the hatches, rod locker for instance, the frame itself is very weak, and very prone to twisting - the paneling gave the resistance to flexing, and it is rock solid) I would use 5/32.
You will be using more rivets than you think you will. You may want to look at buying them online, in quantity. If not, Northern Tool does have them cheaper than Home Depot, but they lack 5/32. Lowes (at least here) doesn't have many sizes, and they are a little more expensive.