There is no 'average' size for a corporate Christmas party. It varies enormously by company and culture. There are patterns, though:
Small companies (up to 50 staff) more often celebrate Christmas together as one group, in a restaurant or at their own location. Mid-sized organisations (50–300) choose an external event venue with a fully catered programme. Large organisations (300+) sometimes split by department or location, or organise one central big party with multiple zones.
Our experience: the happiest staff are at parties where the scale fits the culture. A small company trying to celebrate big, or a large company cutting back on scale: both feel uncomfortable. Match the scale to the expectation.