The key point to note is that the gear ratio of a worm set is not as per the ratio of the pitch diameters of the worm and the wheel, as in the case of Spur and Helical gears. Rather, the gear ratio of a worm set is calculated based on the number of teeth on the worm wheel and the number of starts (threads) on the worm pinion.
We can change the center distance between the worm gear and the worm pinion to change the diameter of the worm gear without changing the gear ratio. By changing (increasing) the centre distance will have a bigger worm gear & pinion and, of course, more Torque and power capacity.
However, generally, machine designers have available space limitations, load, and speed as known design parameters. So, it is necessary to have the best combination of Power rating, torque, and load to be lifted/moved by the worm gearbox based on these known parameters
Another important point is to select the diameter of the worm gear (wheel) and the worm pinion as per the availability of manufacturing tools. Worm gear (wheel) is manufactured by hobbing, and Worm pinion (shaft) is made by grinding or milling with a disc cutter. So, theoretically in calculations, it may seem possible to have a big worm gear with larger torque capacity, but manufacturing constraints also play ciritical role (cost and feasibility !)
Regarding pressure angle, it should not be less than 20 degrees for single & double thread worm pinions and not less than 25 degrees for triple thread.
In general, case-hardened steel is suggested for the worm pinion and bronze (suitable grades) for the worm wheel.