A
big dick is a
penis that has an
erect size larger than some
arbitrary demarcation, usually about two
inches longer than the
speaker's own
penis. (c.f.
penis envy.)
For the gay male of the species, a big dick is generally difficult to deal with for various reasons. (If the afore-mentioned gay male does not have a big dick, then there are three basic ways to play, with almost infinite variation.) First is manual stimulation (jacking off). Having a partner with a big dick is often a nice thing in this sense, in that you can usually get both your hands around your partner's manmeat, and have a lot of fun. Second is oral stimulation (going down, blowjobs). Needless to say, if you try to go down too far, a big dick can choke you or even make you gag. Third is anal sex (buttfucking). A big dick can, almost literally, tear you a new asshole.
Of course, if the gay male is used to fisting (well, more specifically, being fisted), he can receive anal sex from someone with a big dick without too much difficulty -- just use a lot of lube.
Related nodes: penis envy, penis, dick, oral sex, anal sex, manual stimulation, blowjobs, jacking off, going down.