I do mine at 1600x1200, the size of my desktop. Most graphic artists are migrating to bigger and higher res capable monitors every day.

Larger renders include a lot more detail then pixelly smaller renders. When i do my popup product images for my daz products, I always render 2-4 times larger then the popup, and then reduce that down.. sometimes that will give me a better quality popup image, but the amount of detail that's lost is staggering.

A lot of the best new content now has many effects to get things liek skin specularity, displament for micro detail, and that all can add up to a much more realistic image. But if you reduce that image size down to 600x400, it's going to look like hell compared to the original, and as an artist, you want to showcase the best looking image possible.

I've been itching to get a letterbox sixed larger desktop flatscreen monitor, and if I do that, my gallery renders will probably increase in size again, as I migrate to that.

BTW, a Daz popup image is maybe 1/16th the size of my monitor, just a few inches square , and I always zoom in on them because they are just too dang tiny.

Way too many people take way too many things way too seriously.