@Zinko thank you for the good wishes, and best of luck with your coming adventure with the mixes. I think it was Breeder Steve who used to call his mix the "Adventure Mix", because that is exactly what awaits the grower.
As for seed size, shape, color etc. I understand why seed buyers are focused on the look and size of their seeds - at the time of purchase, it's really all the information they have about their purchase, other than the breeder's information - which too often is more marketing BS than anything else.
But as a breeder and grower, I can tell you that size, shape and color cannot tell you definitively whether the seed will germinate and grow into a strong, healthy young plant. And that is all a seed ever needs to do. The rest is genetics and grower skill.
@Muleskinner and @Pjhunt78 are right. Different strains have different sized seeds, to begin with. Sativas and sativa-dominant hybrids tend to have smaller seeds than indicas and indica-dom hybrids. But not always. In particular with the Cindy and Durban mixes, the Durban-Thai Highflyer crosses have small seeds, I believe due to the Thai influences, as the tropical sativas tend to have the smallest calyxes (i.e. seed pods) in order to survive in their humid environments.
Color, and mottling, is generally related to seed maturation before harvest. So, darker is generally better for ensuring the seeds will be viable. However, seeds mature (become viable for germination) long before they turn dark brown, so it's more of an indicator than an absolute. Any plant that was pollinated on a certain day will contain seeds at harvest that range from dark to light and everything in between.
The real test of seed viability is, well, testing. Every FDM line that is offered is germination tested before being offered, and once a year thereafter. When germination falls below 90%, the lot is pulled from sale. (None ever have, and I am still testing lots from 2002.)
Seeds are natural products, and a percentage of every seed lot are duds - non-viable seeds that are not fertile or mature. We select each seed by hand that goes into a lot released for sale. Then, before it is packaged, each seed is given a good squeeze between the thumb and forefinger. If it crushes, it is rejected. With these procedures, we are able to more or less ensure that the viability of the seeds that reach you.
Ultimately the proof of how good your seeds are is in the growing. So get growing man!