Intro and a Quick Question

I think it's a trade-off between a medium resolution for quick loading (possibly for customers using mobile devices) and high definition to show off all the detail and work that went into the model. Mind you, I don't think anyone will download a model to their phone, unless it's for a specific purpose.

Good luck! I'd say check other marketplaces as well, see what are their guidelines and adhere to them. I usually use full HD.