The T-Rex is 42 inches tall at its highest point, its 138 inches long and it rides on 90 inch wheelbase. Overall width based on the front track is 78 inches. At 5 feet 10 inches tall, I still felt like I had pretty good headroom, maybe a couple inches or so at the very least. I can't comment on what it's like with the helmet though as I didn't have one readily available. The most important thing to note when climbing into the T-Rex is to make sure you remove the steering wheel, otherwise it's hard to get your legs in. To release all you have to do is press a small release button grab the collar behind it with both hands and slide it off. To install it back once you're seated, simply place the wheel back on the column and rotate it slightly until it locks in place.
This example also has the optional protection package which adds the carbon-fiber protective pieces across the entry thresholds, in addition to the inclusion of an outdoor cover.
The seating position is an instrumental part of this unique experience as you sit but mere inches off the ground with your torso tilted rearward and your legs stretched nearly horizontal in front of you. Adjustments are limited to the seat backs and pedal box. The ladder is able to slide forward or backward. You then insert a pin into the metal track rod to secure the pedal box in place. The hand brake is controlled by a lever adjacent to your left leg. General ergonomics are good as everything still remains within easy reach despite the seating position.
You wouldn't necessarily consider something like this to be comfortable but the seats did contour my body pretty well and there's decent padding. The suspension also had just the right amount of give without making it too soft. Again in the performance model all that is adjustable.
As expected, visibility is quite good as there's nothing really to get in your way.