Good luck, if I remember right 18's are as big as you can go and that is pushing it.

If you run 35 series tires you'll be fine with 19's. Im running 18's with 35 series tires and I have plenty of room to move up to 19's if I want... which Im thinkin of doing actually.

225 40r18 is the largest tire u can stuff under a venge that hasnt been lowered
its as tight as it can go without rubbing

215 35r19 is also doable
 
going to 20 would require a 40mm offset with a narrow tire
205 35r20 unless u can move the strut over which will be the problem area

yah, 19s will fit.. 20s are really pushing it. you really have to watch the rim width. generally 20s have a wider rim and they have a tendency to rub the steering knuckle. especially when turning, very hard on tires.
if you plus size your rims right, keep your speedo relatively normal the only real concern is width and rubbing. the degree of lowering, complicates things more. gonna be hard to get a definite answer if you drop.