How many pixels heigh and how many pixels wide should I make a character in the heighist resolution? (can't remember right now) so I don't end up with either a fat midget or slender man as a main character.
(This isn't really a technical question about AGS.)
To answer your question, why not use a foto as reference, put it in another layer and scale it down or something like that?
The size in pixels depends on the resolution; a good way to start is choose the game resolution first (320x240 is recommended for beginners), draw a simple room with doors, then use 90% of the door height.
Or you could take a screenshot of a game that's similar in style to what you have in mind, resize it if necessary and measure the pixels dimensions of the characters.