Andrus,
The aluminum coating on your tent ~may~ work as (at
least 'part' of) a ground system, depending on how large
your tent is and how well connections are made. It
would depend on the frequency of use (Mhz) naturally.
How well it works also depends on several other factors,
such as 'shape', just how thick the coating is (if it's
very thin, such as with 'mylar', you may have a brief
flash and no more coating! LOL).
The size (or length) of the antenna really doesn't
make a lot of difference as far as the 'groundplane'
is concerned, what would work for a 1/4 wave should
work just as well for a 1/2 wave. If your use is in the
VHF/UHF ranges, 'maybe'. If it's for HF, there are
better ways of doing it...
'Doc