If you're delivering 5 mL fluid/kg intratracheally (I would think of that as the best way to get most of it into the lung), the volume itself may be too much (5 mL/kg = 5 microL/g = 125 microL/25 g mouse). We've never delivered volumes above 50 microL intratracheally at once in adult mice, although some sources indicate 75 microL as the upper limit. We've also delivered small volumes (25-50 microL/mouse) intranasally, but it would be best to lightly sedate the mice for this and it could be a little tricky.
The best reference I could find now to support the above (re. intratracheal administration) is http://www.bu.edu/orccommittees/iacuc/policies-and-guidelines/administration-of-drugs-and-experimental-compounds-in-mice-and-rats/ .
My pleasure, good luck! In case you are planning to do i.t. injections and you haven't used this route before, there was a question a few months ago: https://www.researchgate.net/post/Does_somebody_know_how_to_do_an_intratracheal_injection