I love big-leafed plants in the style of astilboides, darmera peltata, and petasites, but I can't think of any that are evergreen in USDA zone 7B -- Sunset zone 5. Any ideas / suggestions?
I can think of very few shrubs (e.g., Fatsia japonica) but not any perennials with really large evergreen foliage. Maybe some of the larger-leafed hellebores. Really big evergreen leaves are rare, probably because big leaves are usually thin (otherwise they would be too heavy for the plant to support) and thin leaves are not generally frost-resistant.
I do enjoy my hellebores, and I have fatsia also, but I really like that dinner-plate look that the darmera and others have. I figured that huge, thin, evergreen leaves would probably freeze in the winter. Bummer. Keep thinking, though...please!
Spotted Laurel has big oval leaves that are green and gold. If you get a female it also has bright red berries. It's more of a shrub than a perennial, but it can be trimmed pretty small, and will grow tightly.
They're leaves are not super-big, though. I'd say six inches long by three wide? Maybe a little bigger but not much. It's no gunnera, that's for sure. Sorry. Hope you find what you are looking for - and that you let me know what it is! ;)
The major reason why there aren't more cold hardy large leafed evergreens, is because of snow and freezing rain. Trying to grow some semi-large leaved plants in Virginia and Maryland demonstrates how badly pounds of freezing rain can damage large leaved evergreen trees easily.
I think Rhododendron is the largest you'll get in 7. Nothing like Astilboides.