How much can I lower a 1999 A4 if i get 19's on 225/35/19 tires? Would i be able to lower it 1.25 or would i have to go with 18's?

 

go with 18"s either way.. unless you like bending rims and driving carefully

and get coilovers, that way you can adjust it to the height you like

 

So how much of a drop could I do with 18's?

 

1.25 should be fine with 18s. You don't have to get coilovers unless you got the money for them (they are gonna run you an extra 3-400 over just buying springs and struts). I just dropped my car 1.25 and it is the perfect drop I think, the car looks great on the stock rims.

 

I agree. the stock audi rims are so nice.

 

I have 18's with the voghtland sport springs which drop it 1.5 and it looks really good for a car with just springs.

 

The diameter of the wheel doesn't have a bearing on how low you can go... you can go just as low on 20's as you can on 16's.
Width is what matters... and offset. I'm 24" from the ground to the fender, running 225/40/18 on an 8.5" wheel. If I were running 225/35/19's... I'd have the exact same sized tire (outer dimensions) so it wouldn't make any difference. Same goes for 225/30/20's.