Key generation

[Tom Nott]

All,

I've just started using GPG and have the following questions: 

What do the characters in the set "+.>^" mean during key generation?

Which are the times during key generation during which munging the
keyboard would have maximum effect? Is random data gathered only at the
beginning of the gen-key process, or all the way through?

How much longer does it take to encrypt/decrypt using a 4096 bit key
than a 1024 bit key?

By how much are the size of signatures affected?

Are these two factors the only reasons why keys > 2048 bits are not
recommended?

Please point me at TFM as appropriate.

Thanks in advance,

-- 
Tom Nott