What techniques exist to create larger sheets of origami paper from smaller ones?
According to this thread on the Origami Forum, you probably want to use some form of adhesive, such as tape, glue, spray adhesive, or methyl cellulose to join the sheets.  It will create double-thickness at some points and texture differences (tape, for example, doesn't fold the same way paper does), and of course seams, so you do want to make sure those seams won't show in a prominent area of the finished model, or that you only used the joined paper for practice, and not a display model.