If you wanted to enlarge would it not just be easier to scan it in and get a digital negative then print on a transparent OHP sheet, that way you can choose the exact size and also tweek the contrast etc perfectly on the PC, I will be doing some cyanotype printing soon (just got some watercolour paper and getting chemicals in a couple of weeks) and I shoot on a range of formats from 35mm to 5x4, im getting a photo scanner to digitise the negatives I get anyway and printing on OTP transparancies will allow a much sharper image vs tranfering from one to the next to the next etc. I guess we will see how it works out