i measured the speakers when i took them out, and they measured at 5x7, but i can can check again. and with the adapter i showed in the posts above, raises the speaker a bit, and makes it fit, there will be baffles behind them.
Well I will say it again, Ford has used 6x8's for over 20 yrs and have never used 5x7's. But I guess you can go ahead and spend your money if you would like but don't be pissed when you see that the 5x7 adaptors don't fit right.
The below info is from metraonline.com, a major player in the accessory adaptor industry.
1988 FORD TAURUS 4DR SEDAN
Select a Category... Antennas Harnesses - Into Car Harnesses - Into OE Radio Harnesses - Speaker Installation Accessories - Audio Kits
Car Deck SpecificationsCar Deck Size - DINSpeaker SpecificationsLocationSizeDepthFront Speaker5.252.5Front Speaker3.53.5Rear Speaker6X83Installation AccessoriesThe following parts are required or recommended for installationKit99-5500Wire Harness70-1770