Its really hell to plan this kind of thing.I had to use this 2 images since there isn`t any other references.
For size I used this quote on wookipedia "Development of the A5 also resulted in a smaller, specialized model, the A5-RX battle tank," - so since A5 is 22m long (cca 61 stud) - I planed everything on assumption A5-R is 15m ...I guess end resault is ok, not too small, not too big, whan you put minifigs it looks like a juggernaut series vehicle - massive and powerfull, but you can feel its small one.
Also color was problem - on reference images it look redicules painted in orange..come on! so i made it in normal republic colors...