How to pick time signature, key, tempo, etc...

[Cordelia]

Thanks, this is good advice.
Unfortunately, I'm using FL Studio (which I'm becoming less and less fond of), so I don't think there's a tap option like in Live.

I'm still using FLStudio5, but it probably hasn't changed, so-
Right click the Tempo box in the Transport panel. Click "Tap", a little faucet icon appears. Click that a few times in rhythm.

[Cordelia]

32 bar progression would be insane.

That would be a long "long melody", for sure. Not unheard of in popular music, just rare these days. "Michelle" by McCartney contains a solid 16 bar progression, I've seen "The Long and Winding Road" written out as a 24 bar melody (and that's not including the middle eight). There are many examples of longer progressions from Randy Newman, Leonard Bernstein, Frank Zappa...
But those composers don't think in terms of bars, they follow the melody through the song. Writing a good melody that unfolds over 16-32 bars is a difficult thing to accomplish, hopefully not a lost art.

[buckshead]

Most "popular" songs consist of 32 bar sections, made up in blocks of eights, often A A B A, but also in 16s. Jazz solos nearly always 32 bars. It would be more unusual for the main part of a of all the old standard to be other than 32 bars for the chorus.