Color can be a factor to look for when dealing with immature, nonviable seeds that have been harvested too early. These seeds will usually lack the fully developed seed coat that mature seeds have and will appear white to dull light grey. If the seed has been allowed to properly mature on the plant, the abscission zone won't look like a mini crater and will be partially filled in with white material. If it looks like a deep crater with no white material, the seed was probably harvested too early and may be non-viable. There are always exceptions, but the above mentioned should be used as guidelines when collecting seeds.
Some sources state that the larger seeds have a better chance of germination and survival.
The size and shape of a seed is influenced by environmental conditions and the plants genetic code. Different strains of Cannabis also produce different size seeds. A very potent plant may produce very small seeds, while a low quality plant may produce very large seeds. The size of a seed should not be of great importance. The seeds you collect need to be kept dry and cool.