Right now there is no option, currently we can order only by selecting the existing size.
You can raise a request in mendix support link :) by choosing the issue as request for info =>
Hope it will be helpful !
To your size question. I have some of the shirts and I can say that the sizes do match my expectation. I don’t feel like they are larger or smaller. But that might depend on the region you are living in (european vs. us sizes are sometimes different for example).