Forgive me for not simply internet searching first.

"The fruit is oblate to round, of medium size, 2 3/4 to 3 3/4 in (7-9.5 cm) wide; peel is light-yellow"

Sounds like one is almost ready to go! Yay!

There can also be a lot of variation of size on one tree.
By the time they start to color up, they are pretty much full size and will not grow much bigger.
They may still get a little sweeter over the next 2 months. We are already picking but we have enough to keep us going through April and sometimes into May (5 grapefruit trees)

The out side peel will get a pink spot on it that to me and smell when grapefruits are ready like any fruit they smell awesome!

Danas, while most red grapefruit develop a pink spot on the surface, as you say, 'Thompson Seedless' specifically does not. It never shows any trace of pink pigment in the flavedo (yellow part of peel) nor the albedo (white part of peel).

VA Canuck -- for us in central Florida, Thompson is often ready by mid- to late-November. But because the maturity date will vary with weather (especially temperature), about the only way you can tell for sure is to try one and see if it is to your liking. Grapefruit can usually hang on the tree for months after they're ready, just getting better and better, so you might wait until they begin to fall, and realize at that point that you could have started picking a couple months earlier.