How do I size ArraySink to receive a DER encoded public/private key

[Matthew Cross]

I want to use an ArraySink instead of a StringSink for the destination of 
my Public/Private
Key generation.  I believe I have to pre-allocate my array for the 
destination so my question is
how do I know the size of the DER encoded output for any given size 
key?  Or is there a way
to make ArraySink allocate for me?

Matt