1. How big are they supposed to be? We know Alpha Trion is the size of a standard Cybertronian. Yet that image we got of Prima shows him to quite large since the Matrix is hilt of the sword. 2. Do any of the 13 have alt modes? I'd love Hasbro to do a mini series on these guys. I know next to nothing of them. Wish we could get a group shot of them from the images at Botcon.
The Fallen transformed into a tank in the War Within series. Vector Prime transformed into a Cybertronian aircraft of some sort in Transformers Cybertron. There has been speculation that they dont transform but I'd prefer them to. As for size, they appear to be slightly larger than a normal Cybertronian (The Fallen in RotF, Vector Prime in Transformers Cybertron and Liege Maximo in the old Marvel comic). I think Alpha Trion is short for one of the 13 so he's around the same size as a normal Transformer.
The Thirteen represent some of the core types of Transformers. There is one with a Beast Mode, a Minicon, a combiner, etc. So yes, they can transform, or at least some of them can. As for their size, depends on the continuity. And even then people will shout that size means nothing in a Transformers series. In Prime it seems they are about Optimus height in general.
The question is who/what is his partner/vehicle/transforming jetpack (although since he is one of the 13 he might have had all three)