In general there's not really a bad time to use streamers IMO...unless maybe there is a massive hatch going on that the fish are keyed in on. The thing that makes streamers so versatile is you can fish them so many ways...dead drift when the fish are sluggish, swing across the current lanes, or actively strip for more active fish. When the water is cooler and there isn't any surface activity (Winter and early Spring) I fish streamers probably 90% of the time. A lot of guys nymph this time of year, but I have better luck on streamers...usually presented just dead drifted or swung and just left to hang in the current. When the water is high and muddy, that's definitely a time for streamers too.
Size: Depends on who you ask...I generally fish smaller streamers (normally sizes 8-12, but I have size 14 Buggers that I fish for Brookies with), but I don't mind catching the little guys too. I think there is probably merit to bigger streamers catching bigger fish though. For your average sized trout in PA, I think size 10 is probably about right.
Color: For Trout, mainly black and olive. Fresh stockies, especially Rainbows seem to like white. Black in murkier conditions, olive in clearer conditions. For WW species (Bass/Sunfish), I like white and chartruse or a comination thereof.
Type: Again a personal preferance thing. Woolly Buggers will cover the most bases probably though, and if I had to pick just one, that's what I'd go with. Most of my streamer fishing is with some variation of a Bugger. Clousers, Sculpins, Crayfish, and others all have their place though too.
Weight: I generally like mine weighted...you want to get them down and then bring them back up as you strip or swing them across the current. Sometimes I'll even add a split shot 8 inches or so ahead of the weighted streamer. For most applications a weighted fly and a floating line is probably ok. If you're investing in an exclusive streamer set up or fishing larger, deeper rivers, then a sink tip line may be the way to go.
Hope that helps ya a bit, and good luck. Streamers catch the most fish per cast of any type of fly IMO and they're a good place to start...you get to practice several techniques all on one cast.