Matt - I use a 16x7 for my vintage Sprite and it works but careful packing is key. It also keeps me from spending more $$$ on stuff I would take to the track that would be nice but not necessarily use. I had a 20x8 for an MGC (same as a MGB but two more cylinders) and found it to be very roomy but that let to tool creep and stuff creep etc.
The 16x7 is easier to tow, park etc. If you don't have to take everything you own with you to the track, I think you'll find it is a nice size.
ez