Hey Jay-R,
as far as I know, you can not do that. The size is one for all of the strategies in the portfolio. If it was possible it would be in the input menu. I am sure they can add that, it is an idea, but that will bring you far away from the diversification.
Because if you increase the lot of one strategy it is the same as trading one expert advisor with higher lots in the account. But if that one starts to lose, it will bring you more losses and the others won’t be able to compensate because they are trading with smaller lots.
The idea of trading portfolio experts or many experts in the account is to have equal lots so there will be risk-diversification.