First of all welcome to the site and the addiction!

He looks good, I can't tell how thick he is, but you want to feed him something that is about as thick as he is. I'm not good with mouse sizes, but he looks like he could eat a rat fuzzy no problem. I believe that would be about the size of a weanling mouse or small mouse.

Are you using a plastic tub for an enclosure? You shouldn't have any problems keeping humidity up in a tub. If you need more humidity or less humidity you can add more holes to the tub or cover them up.

There are some great care sheets on this site and if you have any other questions feel free to post a thread in the husbandry section! There are lots of helpful people on the site.