Mine is just over 21", and he eats two small adult mice (just a tad bigger than hoppers) every week. When I got him he was on one small hopper a week, and when he started cruising around the feed box looking for a second one, we bumped him up to 2.

Most people will tell you to go for prey that is slightly smaller or equal to his girth... mine could eat a larger adult based on his girth, but the time I fed him one the mouse kind of scared him, so I went with two of the smaller ones.

As far as the activity level, he sometimes goes right to his hide and doesn't move for a day or two, and sometimes I see him drinking or randomly cruising his enclosure the same day... I guess it depends on his mood.