This is a good starting point for anything having to do with o-rings. There are charts for both static and dynamic seals that you will want to look at. Read the section on stretch of the o-ring and the resultant decrease in cross sectional area related to diametrical stretching and you will get a decent idea of how you can load o-rings to fit the gaps in the sizes. It is somewhat common to use a smaller o-ring than the ones mentioned in their charts (typically 1 or 2 standard sizes) in a slightly larger ID groove to provide some stretching of the o-ring for seals in dynamic applications.
Parker o-ring design guide