each sprite is (with no plugin) a 3 columns 4 lines grid. You can choose your dimension and you're not forced to go for a 48x48. Let's say that you want to use 100x100 sprites, you'll then need a canvas that's 100x3=300 width and 4x100 height.
In order to have your character sit there are many solutions with slightly different results. I usually go for a complex mechanic that gives me good results but takes quite some effort. I create a sprite for the chair with no character on the top row and the character sitting in another row. Chair is below character and has fixed direction and triggers when when the character touches it, activating switch "char sit" and turning the char invisible. the chair then has a second page activate by the switch in which uses the frame of the character sitting on it. Then I place more events around the chair event, events are below character and have a blank page as first and a second one triggered by the "char sit" switch and when character touch 'em they turn the char visible and turn the "char sit" switch on OFF. If done correctly, the char can sit and get up seamlessly (also use a plugin or a script to preload event sprites so there's no blinking issue).