1920x1080, nearly everyone here is doing 1920x1080 templates these days. I just took a quick look at the recent AE templates and I can safely say that over 75% of them are made in Full HD resolution. So you shouldn’t go below 1080.
4K is an option as well, but I don’t think it makes much of a difference in terms of sales. But if your gear is capable of doing 4K, give it a try and see if it’s worth the time and energy. But don’t forget to include a 1080 version for those who can’t run 4K on their machines or simply don’t need it just so that you don’t lose them.
Frame rates shouldn’t be an issue as long as there are no pre-rendered assets in your template. Say your template is set up at 25 fps, as far as I know your end users will be able to render at whatever frame rate they wish e.g. 30, 24, 60 etc.
Hope this clears things up a bit.