I wish my cutting table was large enough for two rotary cutting mats! The larger you can have it, the better, when it comes to doing large projects .... or squaring up quilt tops and working on borders, etc.
It is higher than a standard table which makes it much easier on my back. Actually I wouldn't mind it to be a little higher still. Height would be variable to the person ... are you 6 ft tall? or 5 ft? etc! .... what height is comfortable for you to work without bending and stooping?
I like the leg room under mine ... often pull up a bar stool to it and work at other things because I have such great lighting overhead. There's still space underneath that I can stack things behind where my legs are when I sit down.
Built in storage underneath is a great plan .... consider all the things you need at the cutting table and plan for it to be stored inside, rather than on top! If you have the space available, I would make the cutting table as an island, so you can cut from all sides, if you wished. Plus it gives you lots of space to work with yardage or a whole quilt top. Also, I'd have it on lcoking wheels, so I could push it up against a wall and pull out when I wanted to.
While you're doing updates .... if you don't have it, plan for a design wall !!
Have fun with all your planning!!!