Pretenders always puzzled me with their sizes, are they supposed to be human sized? But in their pretenders shells Cloudburst and Landmine (marvel cont.) are bigger than Darkwing and Dreadwind's powermasters. So confusing =(.

Anyway no matter what, Grimmy looks very silly in his pretender robot mode x)... Actually his pretender shell looks silly too.

There were different solutions in different continuities. Hasbro didn't really give us any indication what they had intended, apart from the 1988 box art where the Pretenders were portrayed as being robot-size.

That single clue may have been what made Bob Budiansky and Marvel depict them as ten-meter tall warriors as well, regardless of how that ruined the "disguise". The Decepticons were made ridiculously ignorant to miss this fact: Bomb-Burst notices that "they're awfully big for humans", and Scorponok (who really should know better, being binary-bonded to a human himself) simply responds that it "only means their cowardice is greater".

The Japanese had a different interpretation though, as you may or may not be aware. In the Masterforce series, where the Pretenders didn't consist of a robot inside a shell, but the armoured human was a third form in addition to the robot and vehicle modes, the Pretenders shrank down to human size in this third form. Or at least the Autobots did; I don't think the Decepticon Pretenders were quite that short in monster mode.
Of course, in Japan very few Pretenders showed up at all. They got four Autobots and three Decepticons in the Masterforce series, plus two of the Mega Pretenders in the Victory series (where the size issue was simply ignored, as the inner robot forms never appeared and the transformable outer shells had been redesigned to look like robots). The four Pretender Classics (the group including Grimlock) were released in Japan, but without the shells.

An note-worthy detail is that Dreamwave had planned to mix the American and Japanese interpretations for their Pretenders. The robots and the shells would have been separate entities like in the West, but the shell would also have been able (both on its own and with the robot inside, I assume) to shrink down to human size. They were even supposed to be able to shed the shell's outer armour, like in the Japanese version, to allow perfect infiltration into Earth society. This was revealed in the fact profile for the Pretenders in More Than Meets The Eye # 8.

This was well done! "Me fallen and can't get up!" Pete, are we to get a sequel for this, "The Revenge of the Fallen"?