Well, I strongly doubt your problem is your speakers. Many Lexus cars have more miles than yours and have good, working speakers. With your speaker net work just failing at random it has to be something in the head unit or amplifier.
Don't put any more money into speakers. Check with you local Lexus dealer and see if they can at least diagnosis the problem and what it will cost.
The sub speaker is 8" inches, and the door speakers are 41/2" inches. The exact size of each speaker can change from model year, and interior lay out. If your IS has the small round tweeter on the top of the door, are 1"inch .
Paul