Am i too [big, small, old, unfit] to do CrossFit?

NO. Everyone is able to do CrossFit because every 'work-out' we do is scalable.

example;
Workout named 'Diane', 21 - 15 - 9 reps for time of Deadlift at 100kg, handstand push-ups.

Depending on the individual we would reduce the deadlift weight to where they're comfortable, even use a pvc pipe to perform the movement. Use rubber bands to reduce the individuals bodyweight to help with handstand push-ups, just get them to hold the handstand position, have them place their feet on a box to reduce weight, substitute exercise to a shoulder press using weights that are comfortable for them, etc, etc.

We will 'sub' exercises in any workout when needed, as long as those exercises maintain the purpose, functionality and stimulus that was intended by the original.