Can I test position sizing methods?

Yes. You can write rules which control the number of positions
(number of shares for stocks.) You will have separate rules
for long and short positions.

When you backtest with a list of securities SpiffyCharts steps through each of your securities on day 1, then through each of your securities on day 2 etc. Profits or losses on Day 1 are immediately available on Day 2 to all securities. This is sometimes referred to a "dynamic reallocation," which sounds fancy enough but it is just the way you will naturally trade a portfolio of securities.