What size battery is recommended for a RYOBI Rotary Hammer Drill?

It's recommended to use a 4Ah battery or higher for optimum runtime and performance like the:  

18V ONE+ Compact 4Ah Battery (RB1840C)  
18V ONE+ 5.0ah HP Battery (RB1850X) 
18V ONE+ 8.0ah Lithium+ HP Battery (RB1880T)

Smaller batteries can be used, but often they will have reduced runtimes and performance levels.