Harness sizes

Do climbing companies usually have the same type of fit in different styles. For example, BD momentum and solution- do they fit the same, way different, or slightly different? 

Yes generally companies will use the same sizing chart across their line. But from one company to another there is no standard, every company seems to have their own sizing, just like climbing shoes.

GearX.com wrote: Yes generally companies will use the same sizing chart across their line. But from one company to another there is no standard, every company seems to have their own sizing, just like climbing shoes.

What are the different style names for then? Like for BD there’s the momentum and solution that I mentioned, is there a difference in sizing in them or is it just stylistic differences? Sorry for all the questions, I’m still new to looking for harnesses. 

Not only are there size differences in harnesses, but there are also differences for what type of climbing you are doing.  I have used BD in the past, but have switched to Petzl.  There is a lot of differences, so a quick read and search will answer most of your questions.  You can start here ... https://www.rei.com/learn/expert-advice/climbing-harness.html 

Different harnesses have different uses, some are better for the gym/sport/trad/ice climbing. And others are just different preferences. For instance the major differences between the two harnesses you mentioned:

Momentum - Adjustable leg loops and a haul loop - geared more for trad climbing
Solution - set leg loops, no haul loop - geared towards sport climbers

Check out this video: Buying a Climbing Harness: What You Need to Know
Here is a write up with different brands sizing charts: ​How to select a harness​​​

While the sizing is the same within a particular company, the fit will be different between models.
There will most likely be sizing differences between different companies.

As others have said, sizing is the same within each brand ( e.i. the length range of the waist belt corresponding to size S will be the same for all BD men's harnesses, but different from BD women's harnesses).

But the fit might still be different, even if the length range is the same. E.g. a harness that is geared towards all day/big wall might have wider or more padded waist belt designed for all day hanging comfort. Which may not work for you if you have a short torso, as the harness would be digging into the ribs... even though the exact same size in the same brand on a sport climbing harness might be just your thing.

Which is why everyone says to try/hang in the the harness before buying.