I was recently commissioned to paint a flower drip painting to a customer's custom size. After discussing with the customer how he intended to get the canvas stretched on to a wooden frame, we decided on an un-stretched canvas size of 244 cm x 93 cm.
I agreed to try to incorporate a larger flower head section in to the painting to take in to account the 'cropping' effect that results from the canvas being stretched over the frame. This cropping effect depends on how deep/thick the wooden stretcher bars are and how much canvas fabric is used on the back when it is stapled to the frame. If you are planning on using a picture framer to stretch the canvas on to a frame for you it is best to discuss these things with the picture framer first.