I made wipes to fit the container I was using, I think I went with 8" square. I just cut old tshirts with pinking shears. They still look fine after almost 6 months of use. I had planned on sewing cute cotton prints to terry, but it wasn't worth the effort for me at the time. If I ever get around to sewing my diaper clutch I will make some to match
As for clothes it really depends on the diaper. She wore newborn sleepers til 5 weeks with nb AIOs, but I had to use 0-3 a bit earlier for bulkier types. I had 3-6 month jeans for LO that she could wear over a prefold or bulky fitted and cover at 2 weeks (with the legs rolled up). She could still wear them at 5 months with a fuzzibunz elite. I have to go a bit bigger on sleepers now because her night diapers are pretty big She will wear a 3-6m over a day diaper, but needs 6mo for night.
1. 8" X 8" for the wipes. I have both 1 and 2 layer wipes and both are fine. I would do 1 layer if you plan to serge the edges and 2 layers if you'll be turning & topstitching.
2. DD was 7 lb 5 oz at birth and wore NB size for about a month. She started fitting 0-3 month size at about 3 weeks old.
1) I didn't make them, but I bought single ply flannel wipes on etsy (from Tender Bottoms) and they're 8" squares. I have some other wipes I got from a friend that included some double layer terry+flannel (I think) and I like the terry side for days when DD has a thicker poop. The single ply flannel work fine though. 2) DD was 8lb5oz but dropped to 7lbs1oz and it took her over 4 weeks to get back to birthweight, so she wore nb sizes for 8-10 weeks. In fact, my brother got her two onesies from Threadless that are nb sized but because they're long and skinny like DD she can still wear them (she's ~12lbs I think).