The Royale in Boston is decent sized: I'd say they could probably fit 500~700 people in it comfortable, possibly more. I saw Sleigh Bells there earlier this fall, so I've been there myself. The Royale also has a sister venue in Cambridge called the Sinclaire that's much much smaller; I would say they could get over 500 people in there unless they're completely booked. I saw Charlie XCX and 2 other no-name groups there (one rock band, the other a starting hip-hop artist).
I believe that's more of a lower-mid-tier venue here though. Bigger acts tend to play at places like The Hard Rock Cafe or House of Blues (AFI, Fleshgod Apocalypse, Avenged Sevenfold, and Eli Golding are a few recent / going to be coming here performers I can name off the top of my head), where as huge acts will more than likely use the TD arena, which is where the basketball / hockey team plays (fairly certain Lady Gaga is going to play there in the near future).