Chapter Roma hotel

European rooms are likely to be smaller unless your US-hotel experience is in Manhattan. Center-city real-estate costs in most of the popular tourist destinations in Europe are very high.

You might look at the Chapter Roma reviews on booking.com. I've found reviews on that website to be pretty accurate. You cannot leave a review there unless you've stayed at the property. The rating for Chapter Roma is high--as I would expect for a place costing 400 euros a night (double) for the random night I chose in early May.

European rooms are likely to be smaller unless your US-hotel
experience is in Manhattan

Exactly. LOL, I'm looking a highly rated boutique hotel in NYC with king rooms that are essentially the same size (178 ft² /17 m²) as the solo room at the Chapter (17 m²/ 183 ft²). What with the smaller bed, you're very likely to have more floor space than those rooms. Our classic double in a nice Rome hotel some years ago was only 16 m²/172 ft², and their single rooms were only 15 m²/161 ft². So really, depending on how it's configured, you're looking at roughly a 15' by 12' room; not all that tight for one person. If it's too small for you, then upgrade to a double. Figure as well that you're going to spend little time in that room; what is interesting about Rome will be what's outside of it! :O)

It looks like a VERY nice property; ratings on booking.com are very high.

In general, Italian hotel rooms, especially in city centers, tend to be cozy. Some of them, like the Chapter, are in old buildings with established room sizes that are not easily changed or not even allowed to be changed. Diminutive rooms-for-one (not really all that common in the US) offer single travelers a little less expensive room when willing to work with a little less space, and can fit nicely into parts of those older buildings Other properties may keep the sizes small for economical reasons (yes, historic center real estate is very expensive) so that they can offer more travelers a more affordable range of rooms.

It shouldn't be an issue in a really nice property like the Chapter Roma hotel, but one advantage of tiny rooms is that they are more easily cooled in the summer by low-end air conditioning. I mostly travel alone. I don't mind a small room at all, but I admit that opening the door to one that's unexpectedly large makes a nice change--as long as the air conditioning works well enough to cool it.