While preparing for another attempt at visiting the thermal areas of New Zealand and Iceland, as well as the western U.S., it occurred to me that it shouldn't be too difficult to compare the sizes of the various area. In The Transactions Vol. 7, T.Scott Bryan does a comparison of the size of some thermal areas. He just presents some basic line drawings, while I figured I could download satellite maps set to the same scale, which would give a lot more details.
The two areas I couldn't include completely are the Upper and Lower Geyser Basins. They are huge compared to everywhere else. Instead I limited them to Geyser Hill/Old Faithful areas and the Fountain Paint Pots. The only other thermal area presented here that doesn't quite fit is Waimangu, where I wasn't able to include the lakeside thermal area.
Update: Added Mickey H.S., and Steamboat Springs.