Is "right-of-way" based on aircraft size?
The size of an aircraft plays no role in identifying who has right of way.

The general rule is that for same-class/type (powered or non-powered) aircraft, the aircraft coming from the right has right-of-way and should be yielded to. This is irrespective of the size within the same type, but a question of manoeuvrability. For different types of aircraft, a powered aircraft will have to yield to non-powered aircraft. For more details see the related question below, ICAO Annex II, FAR 91.113 or another explanation in the Skybrary.

Technically, this means that where separation between IFR and VFR traffic is not provided and aircraft on both flight rules can operate, a Boeing 737 would have to yield to any powered or non-powered aircraft coming from the right.

In most jurisdictions you will find a passage that also details that aircraft should give way to other aircraft that are established on final approach, so a strong situational awareness in the vicinity of airports and airfields is required. Whether separation is provided between IFR and VFR within a traffic pattern, is based on the jurisdiction and the airspace class. In most cases of smaller general aviation aircraft and larger/faster general aviation or commercial aircraft operating at uncontrolled airfields, the smaller aircraft will yield to the faster traffic already on the final approach path out of courtesy or due to good airmanship. It is easier and also cheaper for a smaller general aviation aircraft to perform a 360° turn than for e.g. a Learjet on final approach.

See related question: Which way should you turn to avoid another aircraft?.

See also this YouTube video: Airplane Right-of-Way Rules - Author: Humble Aviation