How strong would a druid need to be able to polymorph into Great Wyrm?

[Sordahon]

Gold Great Wyrm Colossal dragon and Epic Prismatic Great Wyrm for a few hours or semi permamently.

[Eldan]

There's an epic feat that allows you to take a dragon form of any size, so you'd need that. Plus Colossal wildshape, also an epic feat. With prereqs, that's already wisdom 30, 30 skill ranks and 3 epic feats.

You also can't polymorph into something with more HD than your class level. Meaning for a prismatic great wyrm, you'd have to be level 78, or find some trick to count as much higher level for HD limitation.

[Eldariel]

The HD limitation is the main thing. Otherwise Shapechange does this, but as it's been errata'd to max CL instead of max 2xCL, you'd need Reserves of Strength to break the CL cap and then caster level of 41 for Gold Dragon and the mentioned 78 for Prismatic Dragon. If you use Wildshape instead, you need Hit Dice and to count as a Druid of that level; Hit Dice are easy to pump but your effective HD not so much.