What tire size can I put on my Subaru?

You can put MANY different size tire/wheel combos on your old For. The OE 15" is quite a small wheel by modern standards, and indeed offers no choices in lower profile tires to improve handling and grip. If you can find some 16 or 17" wheels cheap I'd go that route first, then either mount appropriate lower profile tires to match OE diameter, or drop up to 1", as you don't want to screw up the odometer calibration too much. But you have to be careful with tread width, as a fatter tire will hit the strut bodies. This is where it gates tricky. If I recall, going to 215, or even 225, may be okay. I LIKE 215mm as an optimization of contact patch with these old Legs/OBs/Fors. I'd try 215/65R15 to drop about a half inch and provide much better handling. But 215/60R16 or 215/55R17 are much better; the ubiquitous later 225/55R17 might be a great fit if there's no rubbing, as used in later OBs with great success. 235 is just too much drag for this old beast.