External Speaker

[kuvash]

I have an Epiphone Regent 30 Acoustic amp.I am happy with it for what it does.At times I use it for a particulsr electric"blues" guitar,I like the sound better for this application that my other two amps(for this particular guitar). At times I have wanted to use an external speaker,this amp has a jack plug outlet for this(1/4").My question is, how large of a speaker(8 ohm) can I use? I have a 10" speaker that I changed out from a Fender amp. If I build a cabinet should it be open or closed? Is there an optimum size for a speaker cabinet with regard to the speaker size? With the external speaker plugged in, will it kill the four 5" internal speakers?

thanks kuvash