Thanks to a post on the Facebook page Transformers Overdrive, we've got an in hand image featuring Siege Astrotrain and Spinister. They're arranged to either side of Siege Voyager Optimus Prime to demonstrate the relative sizes of the new toys.
As was probably expected, Astrotrain's head height looks to be roughly the same as Shockwave, or a hair smaller than a standard Voyager. Though, unlike Shockwave, Astrotrain does look reasonably substantial in proportions relative to its height.
Spinister meanwhile appears very much like it might be joining Ironhide and Impactor in the ranks of Deluxes that are pleasantly large within their size class. While not quite as chunky, Spinister does come up to just about a head shorter than Optimus and Astrotrain.
Astrotrain's size may be a bit disillusioning, but as it is with most everything in Siege, Astrotrain being slightly shorter than Optimus (and Megatron, not pictured) is consistent with the G1 character size reference sheet. Though when you consider his role as troop transport, it's easy to forget how tall he was supposed to be as a robot.
I'm not sure why people would be disappointed. Siege Leaders have always essentially been Voyager Class figures with a packed in upgrade/armour kit.