There are several different things that go into determining the feed size for a subpanel:
Conduit - That sets a maximum size on the wire. 1-1/4" PVC 80 (you didn't specify 40 or 80, but likely at least some parts are 80 for damage protection) can easily handle 3 x 2 AWG aluminum + 1 x 6 AWG aluminum, based on this Conduit Fill Calculator. That's enough for 90A.
Wire Size - Aluminum is physically larger than copper but generally much less expensive. Using that 1-1/4" conduit, you could get more power by switching to copper - 3 AWG 100A or 2 AWG 115A. But you likely don't have that much power to spare.
Load Calculation - You actually need two load calculations. The first is to determine how much power your home currently requires. With 200A service (which is typical), you might have a Load Calculation from anywhere well under 100A (especially if you have gas for cooking, water heating and space heating) to well over 100A (all electric everything, large HVAC, etc.). So you need to figure out what you currently have, subtract that from 200A and see what's left.
The second Load Calculation is to see how much you need. You probably don't actually need a lot. A hot tub (heater, pump, etc.) might use 30A @ 240V, possibly a little more. EV charging is extremely flexible. You can provision most EVSE for anywhere from 20A @ 240V (minimal, but actually sufficient for typical driving) to 60A @ 240V, and sometimes even more. 30A @ 240V provides enough to add around 20 miles per hour of range according to Tesla, though your mileage may vary.
That means 50A or 60A may be enough capacity to do everything you want to do. I would work in a little extra capacity - 20A @ 120V x 2 (but wired so really 20A @ 240V) will provide enough extra for you to have a couple of receptacle/lighting circuits for tools or whatever.
Note that Load Calculation has a very specific meaning in the NEC. It is not (except with purely fixed loads that can all be on at the same time) adding up power requirements of the devices. It is not adding up breaker handles (that is usually too high). It is not looking at your electric bill (for a bunch of reasons). It is more complex, especially for a whole house, but it is required for major changes like this.
- Panel Size - A panel bus is rated for a particular capacity. That may actually be much higher than the included main breaker if it is a "main" panel. Obviously if you want to have close to 100A of loads and a 100A feed you need at least a 100A panel. But you can use a 200A panel with a 100A feed. You can even use a 200A panel with a 50A feed!
But wait, what about that "main" breaker? In a subpanel then "main" breaker is purely the required disconnect. (Required when a subpanel is in a different building from the main panel.) You can have a 200A "main" panel with a 50A feed and 90A-rated wire. The key is: disconnect size >= feed breaker, wire size >= feed breaker.
So now that you have determined how much wire you can fit (easily enough for 90A aluminum), how much power you can spare (based on Load Calculation of utility service), how much power you need (must be <= power you can spare), you can figure out what minimum wire size you need. 2 AWG aluminum is perfectly fine for anything up to and including 90A and is often the best price, or very little more than some smaller sizes. It is actually better to oversize a bit, within reason, as then you can upsize later by just changing the breaker - e.g., if you make changes in your home that result in more power available for the subpanel.
Typical aluminum sizes:
- 6 AWG = 50A
- 4 AWG = 65A
- 2 AWG = 90A
If you see something that says 2 AWG is good for a full 100A, that's when the entire service is 100A, which does not apply here.
