Technically, it's the sensor size that dictates dome size requirements, not the presence or lack of a mirror, and mirrorless cameras range from one-inch all the way to medium format. A Sony A7R III needs the same dome as a Nikon D850. That said, the mirror box on DSLRs tends to make the housings, with all else being equal, about twice as thick as a similar mirrorless camera housing. I had my Sony A6300 next to a Nikon D90/Ikelite setup, and the latter was just massive in comparison.
As far as housing manufacturers go, you basically have three tiers. At the top, there's the metal housings from Nauticam, Subal, Aquatica, Sea & Sea et al. Top quality, ergonomics, lens compatibility, accessories, 100m or even deeper depth rating, the works - but for a price, which can reach five figures for a full setup. Kind of aside, there is a unique offering from Easydive - instead of mechanical linkages penetrating the housing to press and spin all the camera's buttons and dials, their housings connect to the camera over USB and function as an electronic remote. This allows them to cover most of the market with just a handful of models, as all you need to fit a specific camera is a mounting plate so that the lens lines up with the port.
Next, there are the plastic housings - mostly from Ikelite, Olympus (for their own cameras) and Fantasea. Cost is somewhat lower than the metal housings, tradeoff being shallower depth rating (typically 40m) and poorer ergonomics. Ikelite is a particularly bad offender here, as their housings all tend to be built along the lines of the same square-ish box with holes for knobs, buttons and levers.
Finally, there's Meikon. Also plastic, for years they've been building no-frills housings mostly for compact cameras at bargain-basement prices, but in the past couple years, they have started to move up-market with interchangeable port housings and an increasingly broad range of accessories. While their 'good' housings only cover a fairly small range of camera models - Sony A6000/A6300/A6500, A7 II series, A7 III series, A9, Panasonic GH5, Olympus E-M5 II, Canon EOS M3, Fuji X-T2 - the price/performance ratio is hard to match, even on the used market.
For Nikon D7200, your options are basically Nauticam, Easydive and Ikelite. The former two will be very expensive ($3300 for Nauticam, €3000 for Easydive), while the latter is somewhat cheaper at $1700, but is also a giant box - and don't forget that you need ports on top of that, which can easily double your investment. Plus, besides the housing, you need strobes and/or lights, tray with handles, arms, possibly floats, clamps, lanyard or strap, protective covers, spare batteries for everything, spare o-rings, a bag to haul it all in, etc, etc.
My personal choice is a Sony A6300 in a Meikon housing - it's reasonably compact, reasonably priced, and it covers my needs both as an on-land vacation camera, and as an underwater setup.