With regards to the position size limits, they are set by the exchanges and regulators, and depend on the product liquidity. For example, the ES position size limit is about 100,000 contracts, from what I can remember.
With regards to the day trader rule, you need to specify which one you refer to. There is one for IRS purposes, one for the purposes of determining the minimum account size, one for determining the margin, one for determining your commission rates, and probably other rules, as well.