I once ordered a similar product from tranzwear (a STP Mr. Limpy XS, nipple style) and I could pee out of it, though it personally felt too awkward for me to even consider using at a urinal due to the setup. It was a little harder packing with it too due to the thick tube but I think you should get an XS because the balls on the 5 incher are too flimsy (a common complaint), and the tubing in the shaft for the STP could make the shaft a bit heavier in regards to gravity and make it topple over if not secured in your underwear. That's my personal opinion, though..
Also, I don't recommend wearing boxer briefs when packing since they sound far too loose and.. your packer could fall out lol. But if you can't opt for a packing harness I suggest buying either a tight pair of briefs or boxer-briefs, or buying "packing underwear" from tranzwear or any other FTM store online.
If you plan on getting the 3.5 inch.. A good tip is to NOT put the shaft directly over the balls because in some pants it gives off the most obvious bulge. Just try to put the shaft to the left/right and down so it's not directly over the balls. The balls are kind of too big for a 3.5 inch shaft and would have been better for five inches but oh well..
I had a 5 inch STP as well (Mr Limpy , nipple style) but I couldn't even pack with it, I felt like it was a bit too.. girthy. But for longer packers, I recommend putting them to the side and down or directly up and tucked into your waistband. For the 5 incher, make sure you wear more secure underwear as the balls (like I said before) are extremely flimsy and might make the packer flip and such in your undies.
Anyways, that's about all I can think of right now.. if you have any specific questions, feel free to post them!
