Spatio-Temporality:

An entity exists only if it is spatially located.

Comments:

This criterion seems particularly appropriate for a restriction of existence to physical entities, or the entities assumed by physicalists. (It tends to exclude those entities which physicists tend to reject.) 

Yet of course, adherents of the non-identity of mental phenomena with (any) physical phenomena will (or should) tend to reject the spatial location as a criterion of existence. So will (or should) adherents of the existence of abstract objects. In general, spatio-temporal location is a problem in connection with many properties. 

(!) You may consider defining a notion of 'physical objects' in terms of the idea underlying this criterion:

x is a 'physical object' iff x is spatially located.

(!) It may be argued that this criterion excludes properties ('red', 'weak', 'long', embarrassing), or some properties ('weak', 'embarrassing'). It may further be argued that instead of being spatially located, 'these' (those which are excluded) are instantiated, or must be instantiated to exist.