Yeah, the PCs recently fought the Tarrasque in my game, and we ended up using one of the Gargantuan Dragon minis, which was successful. In the last session, they fought a gargantuan monster made of worms, which I actually put on some note cards to form a a 6 x 6 base, which was suitably impressive.
Some upcoming events might pit them against their choice of demon lords, two of whom are Gargantuan Plus, so I've been pondering ways to handle them. Turaglas, an ooze dude, starts at 5x5 and can expand throughout the fight. I'm not sure what the best way to represent that would be. Codricuhn, who is trapped in an immense hole and trying to claw his way out of the Abyss - and is literally orbited by small planets filled with colonies of demons - I would probably handle him simply as part of the terrain. Have some places they can attack and from which he can attack from. Maybe have them change and shift during the fight.
In the end, size isn't quite the game-changer it used to be, so you can afford to go with whatever you gut tells you. Want your Tarrasque to be 8 x 8 so you can use the enormous toy T-Rex you found on sale? Go for it. It isn't going to break the game, and if it helps make for a memorable encounter, then you've made the right call.
