Velcro is good when really little as you can make it tight as needed but snaps are better when they get older as they can undo velcro.

eg ds2 is currently wearing green kids osfm during the day with Velcro fastening as I don't have any small itti snap ins. He's not doing much rolling or other moving yet at 4 months so I don't have to worry about him undoing anything yet. When he gets a bit bigger and I have a bit of time to pull out my med ittis he'll go into snaps so I don't need to worry about him undoing nappies too easily.

To work out rise when buying gf night nappies for ds1 I got a measuring tape out and measured from top of bum crack to belly button(I think it was this high at the front; might have been waistline) without nappy on and looked at the sizing guide on the gf website.
I side snapped night nappies as that way I could adjust for around legs better for no leaks over night.