WB overscan and screen size

[Marcuz]

i'm using ClassicWB (pretty much, to this discussion, it should be alike WB3.0) for tyoing work this week.
with winuae you can make your windowed screen size the dimension you like; so, to help me in text pages bigger than the size of the sreen, bot in width than in (but this it's obvious) lenght, i tryied something: i've set the window size to 800*512 in winuae, and then in the prefs of WB, Screenmode and Overscan i've tryed to match the numbers to those. as in Winuae i've set double line mode on, i would expect something along 800*256 but i've found that i cannot have more than 724*256.
you would say that's ok, this limit is what there is, but what i find strange is that in the screenmode, i can set a bigger number for the dimensons and that the page would scroll aven for 2 times 724. so that space is all calculated by Amiga. is it possible to force the overscan size to something more than that then? by 3rd party programs even.
any info? thanks!