So, we have confirmation that the hitbox is not an issue, but, of course, we already knew that.

I've probably spent hundreds of hours in plains bases, and have had them fly through openings, but I've been "bat proofing" my buildings for quite some time and have never had a deathsquito in my bases since then.

I have had plenty of surprise attacks, but never seen one spawn near me. They are very fast, so it might seem like they "spawn in your face", but I would have to see proof of that.