If you haven't already bought the hennesey, Try this first., and
if you absolutely can't stand it, don't buy a hammock. You'd probably be way
happier with a tarptent as TomD suggests, they are very roomy.

If you have, don't fret just yet, the hennesy is a very good choice. - it is surprisingly
roomy inside (at least to me). I get clastrophobic in a bivy - I don't in a hammock.

1) rig it firm, but not too tight.
2) The hennesy has little elastic points on the side - many people don't use these because they don't care how big the hammock feels - you should - since the hennesey
has loose sides, these pull it out a bunch and make it feel much bigger inside, so be
sure to stake the side elastics out.

Oh, and one more thing, the only real dumb thing about the hennesey is clipping the
tarp onto the hammock support lines with the clips that come attached to the line with prussik knots. I strongly suggest you buy some small lengths of light cord or spectra to hang your tarp with seperately from the hammock, so when you get in, your tarp doesn't sag. Finally, it may be the nearness of the small tarp rigged right overtop that bothers you,
so you may want to consider a combination of:
a) rigging the tarp a bit higher (being careful about site selection/placement so you don't get wet)
b) finding a slightly larger silnylon tarp to use, allowing you to rig the tarp a bit higher, but still have protection from the elements.