Whats the biggest rim and tire you can put on a 1984 fiero SE 2m4?

16 inchers would be about as large as you could go on your Fiero SE. With 16x7 sized rims you would be best to go with a 205/55-16 sized tires.

look at Tirerack.com they have a wheel tire size calculator

If you will be racing, check the rules for the class you will be racing in. They usually have limits on how much you can change. In Solo II stock class, the rule is that the wheels must match the stock wheel size and no tread may be exposed outside the body of the car. Autocrossers managed to stuff 245 width tires on the Fiero. They got an exemption on the no tread outside the body of the car by showing that there was tread exposed on the front of the car with stock wheels and tires. If you will not be racing, you don't want to run racing wheels on the street. Racing wheels are made to be as light as possible and one pot hole can destroy them. "biggest" means different things to different people. Wheel diameter is only an issue as far as you can get the tires you want on the car. I have 17" wheels on my 88 Fiero. I didn't want to go with 18s due to the tire profile being very low. If you will be using these on the street, most people want the outside diameter of the tire to match the Original tire so that the speedometer is correct. If you want that the profile gets smaller to keep the same outer diameter and width. Increasing the width without increasing the diameter also requires the profile to be lower. I think that I could have got tires to go with wheels up to 19". This is where the wheel/tire calculators on www.tirerack.com and other tire shop web sites comes in handy. They will generally lock you into the tire sizes that are available that keep your speedometer correct. If you are not planning on the speedometer issue, you need to read up on the spec sheets of the tires you want. I went to 17" because the high performance tires I wanted on the car were no longer available in 15" (to fit the stock wheels). At that time, the profile ratios for high performance tires were getting lower and I would have needed to have 245 width on the rear to keep the speedometer correct but they had just discontinued them in the 15" diameter. You can go with larger outside diameters if you limit how far the tire is allowed to move when you hit a bump. The people on the list at www.fiero.nl can tell you the extremes that they have done on modified cars. It is a group that is heavily into major modifications to Fieros.