Well, there are two considerations: how much metal and strength is lost to rust and how big the chain is in relation to your anchor loads. 1/4" G4 and 3/8" BBB have approximately the same safe working load limit- about 2,600 lbs and 5/16" G4 is about 3,500 lbs.
Most cruisers would use 5/16" G4 for your size boat.
Yes, I agree that rust will expand to about ten times the metal lost. So scrape off a piece of chain that is typical, down to bare metal and measure the link diameter. Then compare against new dimensions.
Note that 1/16" of diameter lost on 5/16" G4 will result very approximately in 35% loss of the original WLL as the WLL is related to link area, not diameter. If you have lost that much then I would replace it.
And finally consider that galvanized chain loses no metal to corrosion until the galvanizing wastes away. So you may want to start again with new chain.
David