Does it really impact your practicing experience that significantly? This is an abstraction over market, it also doesn't have slippage and hundred other things.
Anyway, size is on the feature list, will be added in following weeks.
yes. If you are long something you can't buy more on a dip or sell half on a rally.
If averaging down is the key to your daytrading strategy, I suggest you rethink your strategy. There is a good saying - "never add to a losing position".