capacitance
Capacitance measures how big that charge bucket is, that is, how much charge a capacitor holds for each volt across it. A bigger capacitance is a bigger bucket: it takes more charge to raise its voltage by one volt.
Precisely, capacitance is C = Q/V, the charge stored per volt, measured in farads (F). One farad is one coulomb per volt, which is enormous, so real parts are microfarads (10^-6 F), nanofarads (10^-9 F), or picofarads (10^-12 F). Geometry sets the value: bigger plate area, a thinner gap, and a better dielectric all raise capacitance. Example: a 100 uF capacitor charged to 5 V holds Q = C times V = 0.0001 times 5 = 0.0005 coulombs.
The energy stored is (1/2) times C times V^2. Choosing the right value sets filter cutoffs, timing, and how much charge a reservoir can deliver. Caveat: the printed value is only nominal. A ceramic capacitor can lose much of its capacitance under DC bias and with temperature, so the number on the part is not always the value you get in the circuit.
Doubling the plate area doubles the capacitance; halving the gap also doubles it. A 1 nF capacitor with twice the area becomes 2 nF.
Geometry sets capacitance.
The farad is huge: a 1 F device is a special supercapacitor, not an everyday part. Most circuit capacitors are millionths or billionths of a farad.