I need to replace the front sight of my Marlin 336c with a higher sight (since I have put a Williams receiver sight on). I have a couple of questions.

Is there a standard size for front sights inserts (size of fitting). Or how do I measure what size I need.
How high should I go?
I'm assuming that I can somehow remove the front insert?
Thinking of a Firesight, (if one is made). Any opinions?
Where can I buy front sights?

Thank you.