I want to increase the size of my Rudder and Elevators. I believe I can do this with no problems but, are there any guidelines or thumb rules.

bigger = more power, but also more power to move it. prob is... your surfaces are designed for the job, so, if you make them like a fun fly, the wing section wont be able to cope at the lower speeds your surfaces will, ie, you stall crash and die.
give more of an idea on what plane, and what/why you want to do it.
maybe a quick fix, if you want more agility, move the CofG back a tiny bit, this will make the plane more twitchy and give the same feel of bigger surfaces.
there are rules, such as % of wing area. fun flys can be 25% for ailerons, some have 90% elevators!

It's an older design, Joe Bridi's Dirty Birdy. I will be flying it as an entry level Pattern ship. I'm happy with the Aileron and elevators but dislike the Rudder. I would like to cut down the Fin and add it to the Rudder.
What do you think?

Sure why not, you can allways cut it off or modify to suit your needs.

sounds exactly like what i did to a small plane of mine to make the knife edge better. go for it

Great, thanks guys