im really not sure, but my guess'd be a 20", going by the toms you already have. theres two main sets of sizes that your average kit comes in: FUSION SIZES: 10", 12", 14" toms, 20" bass drum. This size of kit will give you more "musical" sounding toms, and a punchier, more controlled bass drum. ROCK SIZES: 12", 13", 16" toms, 22" bass drum. This size of kit is all about the power, and sounds ace. you do sometimes get a fusion kit with a 22" bass drum, but its more than likely to be 20".