Just Transpose the formula that you've got... if...

Xc=1/(2 pi F C)

then...

C=1/(2 pi F Xc)

Xc is the AC Resistance (Reactance) that you want in Ohms, C=Capacitance in Farads and F is the Frequency in Hertz.

Say you wanted 50mA at 110v 60Hz (way too high for driving a PIC and a TRIAC say for Lamp Control - more like 15mA required), then V/I=R (where in our case R is actually Xc)...

110/0.05=2200

So we know that Xc must equal 2200... therefore applying the transposed formula...

1/(2 x 3.14 x 60 x 2200) = 1.2uF

Theoretically, that's great... but practically you'll discover that Capacitors don't come in all the myriad of values that Resistors come in, so you probably only have easilly available 0.1uF, 0.22uF, 0.47uF and 1uF. Work out which one best suits your current requirements and design your circuit to match it. I would strongly discourage you from desgining Transformerless Power Supplies with Supply Capacitor values greater than 1uF.