DC blocking capacitor value for 2-12GHz?
The value you choose for a blocking capacitor mainly depends on how low you need your low frequency cut-off to be.
Macom chose 100 pF for their demo circuit in order to obtain a low-frequency cut-off well below 1 GHz.
If you only need a 2-GHz low-frequency cut-off, and (say) you're willing to accept 10 ohms reactance from your blocking capacitor at that frequency, then you could use a capacitor value as low as about 10 pF.
But if you are using common 0201 size multilayer ceramic capacitors as recommended by Macom, the value you choose won't affect the performance at 15 GHz much, because at that frequency the behavior will be dominated by the inductive parasitic behavior. And the parasitic behavior will mainly depend on the package size you choose (0201 for Macom's recommendation), not the capacitor value.
The part recommended by Macom will give about 10 ohms reactance at 8 GHz (and no data for higher frequencies) according to Murata's SimSurfing tool. As shown in the Macom datasheet (comparing the S21 data for the bare chip and the demo circuit) this will reduce the gain of the circuit by a couple of dB.
If that's not acceptable, special &quot;broadband&quot; and &quot;ultra-broadband&quot; capacitors are available. These essentially package a very high-frequency low-value single-layer ceramic capacitor with a higher-value multi-layer capacitor in the same package. These allow you to push the low-frequency cut-off down to the 10's of kHz with a resonant frequency out beyond 20, and in some cases beyond 50, GHz.  But for these parts expect to pay a few dollars each instead of a few cents. Manufacturers include Presidio Components, ATC, Passive Plus, and Knowles Precision Devices.