Same sizing as ice skates. Just get yourself a relatively cheap pair of "entry-level" skates from any of the manufacturers whose boots you're used to, already. A modern pair of $150 or $200 Bauers (for example) are much better than the top of the line skates that you could have bought 20 years ago. Same goes for modern ice skates if you decide to go with a conversion.
I hadn't skated on wheels in 20+ years until I dug out my 1997 Bauer "Breakout 50" and used them until my Marsblades were installed on a new pair of boots. In the meantime, I tried and then resold a pair of Bauer RS; the boots were fantastic but I didn't like the Hi-Lo setup instead of smaller wheels in front and in back. The boots I bought for the conversion were virtually identical to them: entry-level Bauer MS-1 ice skates ($65) and they're great. You're not going to break down either of those boots without skating very hard in them for quite a while. If you spend $300 or $400+ on much higher-end skates, you may find them way too stiff, as I did: I also bought a pair of Alkali Revel II and returned them to Hockey Monkey immediately without even skating in them (obviously) just based on how uncomfortable and stiff they were compared with the Bauers, which felt perfect right out of the box and without baking. (The MS-1 are so "low level" that they're not even intended to be baked.) For reference, I normally use very stiff ice skates and tape them heavily for lateral support...so, if I'm saying these boots are plenty stiff, you can trust that.