The party is in the habit of using extra-dimensional spaces such as Rope Trick or Psychoportive Shelter. Recently they want to use it as a sniping blind or bunker.
As per the description of either, people inside the space can see out, but not the opposite. Powers and spells cannot cross the interface, neither can area of effects. They are wanting to stick their hand out and manifest/cast stuff at enemies, as well as shoot or throw things at enemies from inside the space, and then withdraw their hand/weapon so they can't be attacked. (The 5' movement rule is being proposed for that part.)
Both the Rope Trick and the Psychoportative Shelter entrance can become invisible (handy for sniping). The former requires the rope to enter, but the latter doesn't seem to limit who can enter, assuming that you can see the interface once it is invisible.
The Rope Trick interface is a 3'x5' window, limiting how many could use it effectively. However the Psychoportative Shelter interface is a 7' diameter plane, which gives considerably more room, not to mention that it is most likely to be vertically emplaced, unlike the horizontally emplaced Rope Trick.
Therefore, how much of a person or object must cross an extra-dimensional space's interface from inside, in order to successfully attack a target outside of said space?
Please note: this question is not addressing the meta-balance issues of this tactic - it may be quite effective at low levels, but once Dispel Psionics / Dispel Magic comes on the table, it's no big deal. There are also the Transdimensional Power & Spell feats available. Plus enemies can pull the same stunts. So balance isn't the concern. This question is intended more to address the action economy, mechanical rules, and order of events.