Some ideas:
Storm Giant has Freedom of Movement; which you may elect to mean it can move through wall of thorns without a strength check.
The Monster Ecology section of the CKG gives you permission but advises against assigning abilities to monsters, but here it's actually the balancing factor: a higher strength makes it easier to travel through the thorns, and giants should have strength in spades. You could use the god-like attribute table from the CKG for this. For reference, 3.5e d20srd gives them:
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GIANT STR
Cloud 35
Fire 31
Frost 29
Hill 25
Stone 27
Storm 39
(This is probably an awful idea. CKG gives a +10 at 30 and doesn't go above that.)
A simple +2 for every size category larger than medium would work (from the otherwise unrelated Grapple rules), but C&C only has 3 size categories. Still, +2 is something. If you want to use 3.5e sizes to compute this (+2 large, +4 huge):
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Cloud Huge
Fire Large
Frost Large
Hill Large
Stone Large
Storm Huge
I always am curious where all these boulders are coming from. It says they can hurl them, but they have to catch one from a catapult or otherwise have them at hand, in my opinion. I'd say if the wall is below their waist and they have a bag of boulders, then sure, they can throw them. But I'd probably make them make a check if they have to find boulders under the thorns, and not let them do it at all if it's a nice marble floor with no rockslides in sight.