As far as I know, you can't, sadly. =( You should try to fit it in the space they gave you. Try to find some workaround, like in the learn tooltip you can put some of the target info and everything. Then in the actual tooltip you can put the tooltip just for that level of the ability. (unless you already are using the tooltip of just that level of the ability) I don't think the max chars is alterable, Blizz probably didn't want tooltips taking up the entire right portion of the screen. =P