Right on the edge of too tall. Depends if you're mostly torso or legs, I guess.
I've had several and I'm 6' tall. I fit very comfortably. If you have a real roll bar (not a style bar) you may not pass the broomstick test if you're taking it to the track.
A friend of mine is 6'4" and he loves his Miata. Tight fit for him.
I can only speak from personal experience, I'm 6-2' 205lbs I can fit in the first generation Mazda Miata no problem but with the top down the top of the windshield is in direct alignment of my eyesight I either have to lower my head down below it to see out or put my head above to see over it as if I was driving a clown car in the parade. and of course with the up I have to crouch down to look under the top of the windshield.
Either way I find it very uncomfortable and dangerous to drive, I do not have this problem with 2005 and newer Miatta's.
6'4" and fit just fine in my '96.
Pro tip: I removed the inside arm rest and gained about an extra inch and a half of knee/leg space, making it much easier for my hands to clear my legs whilst steering.