6.5 inch all around. However, you would want to make sure you dont have the amplified system in your car. Some GT's do. Best way to find out is check your pass kick panel next to the glove box for a silver box with two harnesses pluggled into it.

The reason being, the amplified system has 2 ohm speakers (except for the tweeter its 4 ohm). Most after market speakers are 4 ohm. If you install a 4 ohm speaker in a 2 ohm system, your only going to get half the volume out of your back speakers. There are dual voice coil 6.5 speakers out there though.

And another point, the premium sound system is internally crossed over. All 8 speakers are powered on seperate channels and each channel is crossed over internally in the amp. The rear speakers in the OEM set up will only recieve midbass frequencies, so installing a 2+ way speaker will do you no good.

OEM premium sound with after market speakers = sounds like crap.

 

so I just got my 01 celica gts in February 2020. purchased pioneer amp, (4 channel) , a 10" pioneer sub, and a pioneer blue tooth am/fm receiver. I just got done re-wiring the stock rear speakers so that I can run them of my amp but separate from my sub. next I'm going to figure out how to use the factory amp for the door speakers and tweeters. hopefully goes well. 🙂

 

Lengthy.... but read the Scosche TA-03 Wiring Kit Noise Mod sticky.

 

so a friend of my husband hooked up my sfereo this way. not good there was every frequency coming out of every speaker. my ears did not like that at all. although some may . but it works to have the speakers go through the amp and deck with this plug and a couple extra MMF cross overs . lol