Read my post.

Never ignore cre he always has outrageously reliable info.

Thought I'd chime in on this post. Last night I just helped a friend install new front speakers in his 1990 Supra. I can confirm that the door speakers are 10cm / 4". This car originally had the factory cd player with amplifier and 6 speakers (doors, rear seats, and hatch).

Interesting note worth mentioning, whomever installed a radio in this car before we got to it did not seem to know about the factory amp wiring, and they ultimately chopped up the harness and ran their own speaker wire. If you upgrade a factory amplified Supra, and intend to delete the factory amp, you can still perform a clean 100% reversible install using the typical 82+up wiring harness adapter found at just about every car audio shop. After removing the factory radio, disregard the harnesses plugged into it. In the back of the radio cubby, there is a wiring harness running left to right that has 2 white connectors. Unplug these 2 connectors and connect your harness adapter. You just bypassed the amplifier and have all of the correct connections for your aftermarket radio to function correctly. Now if you wish to return to the stock stereo, you can do so by just plugging it back in.

Also you won't have complete strangers cussing out your hack job and discussing just how stupid you are as they fix all your mistakes :)

That's correct the size is 4" and it's quite a common size so you wouldn't have problem choosing a fitting speaker for it. By the way, this tech article covers the speaker installation pretty good: Audio Speaker Basics