To know for certain, you'll need to:
* Consult a repair manual: A Haynes or Chilton manual specific to a 1990 Ford Probe will list the exact bolt sizes and specifications for the front motor mount.
* Check the existing bolts: Remove one of the bolts (carefully) and take it to a hardware store for size matching. Note the thread pitch as well (e.g., 1.5mm).
* Use a parts catalog: Online parts stores like AutoZone, Advance Auto Parts, or RockAuto allow you to search by vehicle year, make, and model and will show diagrams and specifications including bolt size.
Do not attempt to guess. Using the wrong size bolt could result in damage to your engine or motor mounts. Always refer to reliable sources for accurate information.