Normally, OMR forms are either A4 or US letter size, and can be either single or double sided. However, there are exceptions, in that these types of forms can be smaller. If the form is complicated and requires a lot of data capture, then it can be A3 folded and perforated, or even extend to being a booklet.
Traditionally, forms are red in colour, especially where the data fields are, as this red colour drops out when scanning, leaving a clear mark on the page. A red coloured form can be filled in using pencil or black/blue ink, but forms printed in blue, green, etc, generally have to be filled in using pencil only (higher carbon content).
Examples of some OMR forms and styles are given below: