@architect - I lied.
My quill scale is 9-3/4 including the scale mounts (measured just now). If I recall correctly, I redid it cuz I didn't like the first install. The required travel is fine as listed above, but it was too difficult to install like that. Most guys don't do quill scales, but I wouldn't give mine up. I love the combined readout cuz you can move both the knee and the quill and not have to reset your zero.
Here are my other lengths as installed (including the end caps)
X - 35", Y - 17", Z - 21"
Note that you have to clarify with the seller that this is with or without end caps and with or without the read head. Usually, you measure your total travel and they adjust for end caps and read head. But it is critically important that you are both using the same definitions.
Lastly, you specified a J-Head. But J-Heads came with different table sizes. I think both are important.
My table is 9x42.
The safest bet is to measure your travels and then let them adjust for the read head and end caps (which also vary).