The number of items usually has more effect on Finder performance than the item's size does. That is, a folder with 100,000 items of 10KB each can slow down the Finder more than a folder with 10 items of 200 GB each.
This assumes you don't have previews turned on, nor the Calculate Folder Sizes feature in Finder. File size can affect both of those features, causing Finder to be slower.
This also assumes you aren't using sub-folders. If you put items into sub-folders, say 100 items each, then Finder may only be slow if you expand a List view to show large numbers of nested items at once. It should navigate through the individual sub-folders pretty quickly.
Finally, the underlying media and connection speed can be important. A DVD or CD will be slower than a typical SD card, which may be faster or slower than an HD. An external SSD on a USB-1 bus (or a swamped USB-2 bus) will lose a lot because of bus limitations, but on a USB-3 bus it may be much faster.
Summary: there's more to it than total size or the number of items.
I once wrote an app for a friend who wanted to archive his photo collection to 100 GB mdiscs. It divided his folders and nested sub-folders into sets of 100 GB or less, and copied them to dmg files for burning. It took quite some time to make the dmg's, because it was running on a USB-2 hard drive and copying ~100 GB of data for each. However, once the dmg's were made, they could be mounted and navigated easily.
I suggest that you test things yourself, or post approximate numbers. "Very, very large" is too vague to make a prediction about.
