You've got loads of options. You could field an army of giants in Dragon Rampant easily enough - each giant would be a 'single-model unit' of a type appropriate to the look of the giant (Greater Warbeast, Bellicose Foot, etc.).
Havoc (by Brent Spivey/Bombshell Games) has some terrific giant rules. The game badly needs a second edition, as the rules are a bit of a thicket, but the game itself is brilliant. So too is the same author's Rogue Planet, which is perfect for giants (whether they're fighting knights, orcs or space marines). It's full of rules for picking up foes and throwing them into terrain, charging into people with colossal force, and so on. Unlike Havoc, the rules are slim, streamlined and well edited - though, again, both games are terrific. I'd say Rogue Planet is one of the very best fantastical skirmish wargames out there - and it's ideal for outsized antics.
Song of Blades and Heroes can handle giants pretty well, too - though if you were fielding lots of them, you'd probably want lots of opponents, as any decent giant would be the best part of a full warband on its own. But it's a great option for a multi-player game - especially if you had the giants taking on lots of unallied factions at the same time. As with Rogue Planet, you just build the profile you want. A giant would certainly have the Huge and Tough rules, but you could vary each one considerably.
I'm sure there are many more!