Lots of variables. To do it right and get pretty close find someone that can do a Manual N on the building. That’s the commercial one. It takes into account commercial style building methods operating the system only at times you choose higher heat loads from work inside and ventilation from dust collectors fume hoods etc. That said if you have it done by a local company they will have insight on IF the sizing it comes up actually works in your area and what their experience has been.

But without being there… Some things to keep in mind. If it’s an insulated wood structure it will be far easier to cool than an insulated metal building. The conventional metal building construction puts in massive amounts of thermal bridging because the insulation is pinched between the skin and the steel framing. Spray foam is better only if it also covers all that interior steel in the same thickness as the skin.
I am in Texas which has the worst overall weather in the country. Not a place you want to live if you enjoy being outdoors.. Now my 20x40x12 insulated steel building has 2.5 tons of AC. It’s a conventional 3″ vertical pipe welded to the slab plates and then a 2×2 welded frame. That is covered by standard metal building fiberglass with the white plastic on the inside. It will stay comfortable as long as you start the ac up early and the temps don’t get over 100* for very long.
However in the heat of the summer you don’t have a chance. It doesn’t cool down at night and for me to use my shop in the summer when it’s 106* daily with the low at night of 80* and it’s still in the 90s after midnight… I would need somewhere around 6-8 tons of AC or more.
Remember this is an intermittent use building with high external and high internal heat loads along with a huge amount of thermal mass in the building that has to be cooled. Houses only need tiny units to cool because they have little thermal mass and you are keeping it at a steady temperature. Shop and commercial buildings need far far larger units.

If your building was here and was a metal building it would easily need 10+ tons.

For unit types it depends on your needs and budget. Minisplits are nice but they are expensive(and efficient) and they have issues in shops because they have no real filtration on them to catch workshop dust. Basically a window unit filter. You can use conventional residential units as they are the cheapest and can be set up with good filters.
There is also the case for package units which I like because they are entirely outside can be filtered and ducted well and they come in one box ready to run. No copper to run etc which is a cost and a liability. They are not as efficient but they are convenient easy to work on and replace. They are the quietest units as well because they are all outside.

Just some info to get you started. Location type of construction type of use etc play a huge role as well as are you going to keep it at temp all the time or intermittently. Two identical buildings side by side could vary in their AC needs by 50% just due to their intended usages.

The problem I had with the Mitsu 6k’s is that they don’t modulate very low, and with my super insulated house would still short cycle. A Mitsu sales manager told me they were coming out with a 4k but I haven’t seen it yet. Thus I ended up going with Daikin VRV Life where each unit output can drop very low if needed.

@Toni, Mitsubishi makes a high-seer 6k ductless mini-split (MSZ-FS06NA & MUZ-FS06NA), also marketed by Trane and American Standard. The smallest ducted mini’s are 9k. You’ll need to get a room-by-room load calc to know what sizes would work best.

I live in south Texas and want to go minisplit. My house is small at slightly over 1600 square feet. I want separate zones for the beds and the largest is like 14×15 and the smallest 10x 12. I have an open floor plan in the rest of the house except for a door for the laundry room and of course for the garage which will end up being finished off as a family room. I figure I am going to have to use to systems. I think 6k would be plenty for the bedrooms and the laundry rooms. I am having trouble finding that size with a system. Any ideas how to do this without oversized it. Besides the fact that these systems I like the fact that the parts are cheaper to replace. I had a big system 17 sear and it lasted one year after its warranty expired(11years). Any suggestions?

I’ve done small ducted minisplits with a regular filter. Just have to design for appropriate static pressure. Not 1/4 ton though! 1 ton