That'd be about the right height to slide double drawer filing cabinets under.
While a 50"x90" surface might be perfect, you might want to think on the long term as to whether it is practical. Eg. if you move ... or if you want to change the layout of your room. We quilters do tend to change things, when we get better ideas, or see that our needs have changed.
Also, it puts it all at one height ... if you are wanting this one table as a cutting and sandwiching surface, ergonomically, the heights for those two are usually higher than for sitting to sew.
You might want to consider separate pieces, that could butt up to each other for one continuous surface when needed, but when apart the legs could be extended or bed risers used , to give the additional height.
Just some thoughts ... and yes, maybe you have thought this thru and know that one continuous is what you want/need.