What is the largest wire size I can use to fit a 1 14 conduit?

This depends on how many wires you need to put through the conduit. Typically you would figure out what size wire you need to handle a given load, and then find the smallest conduit you could possibly put those wires in. I'd actually need to know what type of conduit, also. Size your wire first, then your conduit. If you don't know how to size your wire, call an electrician. If you do it wrong you will start a fire. Once you've got your wire size, NEC Table C1 for EMT or C2 for PVC should head you in the right direction. ----

If you do not understand the work well enough to accomplish it yourself properly and safely, don't try it. Consult a professional electrician, as they are proficient enough to do it properly and safely. When working on electrical circuits and equipment, make sure to de-energize the circuit you will be working on. Then test the circuit with a definitive means to make sure it is off (multimeter with metal tipped leads, voltage tester with metal tipped leads, etc., not a non-contact tester, which is non-definitive.)