A 45' cat is plenty of room for 8 adults. I have taken even 10 adults on a 45' with plenty of room still. 2 heads is also generally plenty as well, while 4 heads is nice, it generally means smaller cabins and much smaller heads. As far as what time of year to go, it is really not much different besides the temperature at night. Even in July you will usually find plenty of wind so I think the time of year is a non subject unless you are intentionally trying to avoid crowds, which summer would be best if that is the case.
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