Dear,
They are usually RF & Noise Blocking capacitors for Power Supply.
for calculating a typical value : Xc=1/(c*w)=1/(c*2*pi*f)=1/(c*6.28*f)
Calculate C to reach Xc <= 0.5 ~ 5 ohms,if you use greater C theorically you can
reach Xc << 1 but greater capacitors usually have lower self resonance frequency
and you can't use them in high frequency.
according to this usually you must use 2 or more capacitors with different capacitance
in parallel for a bandwidth.(Higher capacitance for avoiding power supply noise&ripple
and lower capacitance for RF Blocking).
Good Luck.