No, but seriously.
MASSIVE FUCKING WHALES IN SPESSS!
Some of them are twice the size of Holy Terra. It is commonly accepted that they are, in fact, so large that an entire Tyranid Hive Fleet could not eat a single Void Whale. Once, a Great Company of Space Wolves found a mutated, Chaos-corrupted Void Whale. They killed it (with a few meltabombs to the heart), creating a new asteroid/mineral/gas field. Kaptin Badrukk, Freebooterz Flash Gitz Warboss is noted to have personally slain one.
Of course, seeing how they are warp-born creatures, their size may vary within and out of the warp. And seeing how 40k is rife with inconsistencies, the true size of these creatures are unknown. Seven unfortunate void whales wandered into the Garden of Nurgle, rotting away and being holed out for nurglings and daemons and Plague Marines during Mortarion's merry crusade into Ultramar. Basically, the biological equivalents of an Ork Rok; slamming their bloated bodies on the surface of Iax. Fuck, imagine the smell. The several page description by Guy Haley is some next level heresy, or maybe /d/.
It has been described that instead of being double the circumference of Terra, these Void Whales are the size of Imperial Battleships. Large, but not stupidly big.
As of Battlefleet Gothic: Armada II, recent images seem to be in agreement with the size estimates of Morty's fleet. Void Whales are now Retconned into Battleship size, maybe a little longer than most Battleships but still in the realm of being reasonable. It could be concluded that the whole "Void Whales being larger than Terra" shtick was just your usual 40k hyperbole and flowery language which should be taken with a truck load of salt, or that it was a Void Whale within the Warp, and we know how much the Warp likes to screw with physics and spatial dimensions. So yes, Void Whales are Battleship size, not planet size, though individual titans and extremely old specimens could potentially hit planet and higher.