Determining the capacitor value for an ac ground

I am referencing a LNA linearization technique paper:
http://www.jpier.org/PIERC/pierc17/03.10082411.pdf

It is mentioned on page 33 of the article that,
"It should be noted that the IIP3 is independent of varying capacitance (C1), because the C1 is for ac ground."
for Figure 1.

How is the capacitor value determined?

Thanks,
Tyro

---------- Post added at 18:51 ---------- Previous post was at 18:40 ----------

Nevermind.
Rule of thumb
Xc<0.1R
http://www.wisc-online.com/objects/V...spx?ID=SSE1502