AGMs have about the same power density as Lead acid. So, they will be about the same size for equal capacity.
Point of clarification. Batteries don't keep the bike charged, the bike is supposed to keep the battery charged.
As for sizing, that's about the electrical budget. You need to tally up all the electrical users in either amps or watts.
The Cb550 alternator can make 150 watts when revved to 5000RPM.
At idle it makes about 50. If you never idle, then the total load must be under 150 for there to be any power available to recharge whatever battery you choose.
If you do idle, then the battery makes up any power deficit until it depletes.
The stock bike draws about 120 watts with the lights on. And the system relies on more driving at higher RPM than idling to keep the battery up. It's 12 AH battery. By a crude and inaccurate approximation, it supplies something like 144 watts for an hour (longer time with less drain). So, in theory you can let the bike idle with lights on about 1 1/2 to 2 hours with a new lead acid battery or gel battery and the lights on, before the engine dies because of lost spark voltage.
Starting batteries have two ratings, one is Cold Cranking Amps (CCA) and the other is a discharge rate over 10 hours. I believe the 12AH stock battery can supply 1.2 amps for 10 hours before the voltage drops to 10V. 10X1.2 = 12 AH. The CCA is in the 150-range, though it isn't often stated prominently.
Smaller batteries in the same technology, deplete faster and have less CCA.
If you want the same versatility as stock, best to keep it stock. If you are making a purpose built machine, best to know the intended purpose. Like, how long it can be away from an external battery charger, for example.
Here is an electrical budget max and min I made for a CB550 some time back.
Stock CB 500/550 electrical budget.
Alternator output - 110 watts at 2000 RPM , 150 watts peak
50 W -High beam Headlight
3.4 W -High beam indicator
8W -tail light
27W -Stop light
16 W - 8 W x 2 Front run lights
13.6W - 3.4 W X 4 Instrument lights
49.4W -23 W X2 turn signals plus 3.4 w indicator
28.8 W - maximum for ignition (when points closed, one at a time)
28.8 W – for the alternator field coil (Only when battery low)
225 W -total maximum drawn from battery
600W -Starting motor
--- Minimum lighting ---
40 W -low beam Headlight
8W -tail light
16W -8W x 2 Front run lights
13.6W - 3.4W X 4 Instrument lights
15W - ignition estimate (points aren’t always closed)
28.8 W – for the alternator field coil (Only when battery low)
121.4 W -total (normal electrical draw from battery)
Battery capacity is about 12v x 12 AH or 144 Watts
Cheers,