I’m going to get an old rigid-frame mountain bike for use mostly as a city commuter, and also for gravel road excursions and the occasional road tour. What size should I look for? I’m 6 feet tall but have a relatively short 30” inseam. I consider 58cm/23” my normal road bike size. The MTB sizing recommendations seem very small and they must be calculated with off-road riding in mind: according to one chart I should get a 19” frame. I suppose they have slightly higher BB’s to begin with, but I figure the smaller wheels make up for that. Shouldn’t I look for something larger for road riding? Thanks for your advice.

For road use, a shorter top-tube than your off-road size.

A 19" should work for you - be sure to ride it before you buy.

slacker geometry and sloping top tubes also cause seat tubes to be shorter on most mountain bike frames. listen to sscyco and ride before you buy!

Can't just look at seat tube measurements, nor compare road to mountain bikes on that one measurement. I'd think somewhere in the neighborhood of a 24" top tube would be a better focus point at a guess. Will also depend on what you go with up front to an extent in terms of spacers/stem/bar setup.

Many that ride road and mtb want close to the same posture nominally on each bike. I ride with a 575mm eff. top tube on the road and 625mm on dirt which is a Trek 19.5. The reason for the difference if opting for essentially the same position can be easily explained. With mtb handlebars, they sweep slightly back from the stem clamp centerline where as on the road hood position is about 80mm in front of the stem clamp center. Also, most ride with their bars a bit higher on the dirt. Rule of thumb is mtb's have a 4-6cm longer tt compared to an equivalent road bike size.
HTH.

30" inseam is going to be your problem. Thats My inseam and I am 5'6" short. I ride a variety of sizes- but you will have to look at the convention of Mountain bikers using smaller frames and longer seat posts.

Do not go by Frame size to your height- Sort the toptube as being comfortable and fit a longer seat tube.

Ok- I am extreme but I ride a 15" as my main MTB. I also ride a 51 on road bikes But On mountain bikes I also ride a 17" and a 19" Different manufacturers with different Geometry. And don't go for the same ride position on an MTB as a road bike. Road bikes do have a lower more stretched out riding position. Attachment is of my MTB set up for offroad. For road use I fit slicks and Flip the stem to give me a lower bar position.

30" inseam and 6 foot? Is that why you are using a MTB on the road? Since you need the sloping top-tube and increased stand-over?

Guess you have never seen Lance Armstrong ride a mtb. He won in Leadville this year riding his 19.5" Trek Fuel dually with 625mm top tube and 120mm stem. He is likely more stretched out on his mtb with hands on his grips than he is on his Madone with 580mm top tube and 130mm stem on the hoods.
Fast riders on either mtb or road like aggressive positions on each.

I am probably one extreme- Lance may be the other. On top of that- His road riding position on a bike would make him feel hunched up on an MTB.

But When i went road -I had a choice of two frames to fit me. Went for the smaller after years of Mountain biking as it felt More comfortable. Even raised the bars to get more comfortable. Not so on the 2nd bike once I had got used to road riding. Saddle 4" above the bars and that long stretched out position. Just try riding your MTB over rough terrain with your head between your knees as you do on a road bike. Doesn't work for long.