The criteria for advanced is theoretically two paragraphs, which honestly has never really been enforced and RP management is up to the GM. I think the suggestion of putting the post criteria in the interest check is a good way to communicate up front. 

In an effort to keep player engagement up, I told people in an RP that a three paragraph post was the target to keep the plot moving, because I'm tired of people thinking they need to shit out a novel or something when the plot just needs a few paragraphs to describe reactions. I'm also tired of dropouts from the RP's. 

Sometimes a much shorter post is totally fine in an advanced RP, and maybe that's where communication comes in. Some people look at it as an effort thing, but I look at post length as a matter of not jumping too far ahead on the interaction if you are in an interaction situation. That first post is often going to be long, but it isn't necessarily the yardstick for the rest of the RP.

Anyway, I feel [@Drache]'s pain; I look at the 1x1 interest checks with a million rules/restrictions and a list of pairs and go, "So I gotta put a dog collar on and do all the plot development? Gee, thanks, pass." 😀