I've tried the 10% and 300% on vanilla but it gave me very small islands within a huge ocean. i've done some testing on sizing with multiple different worlds. I've found the closest thing to it is 30% and 300% I flew around to make sure its a continent along with how far you have to find others. main land was pretty large yet heading east was even bigger. when I went north I foundd two smaller continent that were close together but still decent size.
If you do spawn on a very small island its probably just bad luck and a bigger island or continent are probably somewhere else in the ocean.
Two images is of mainland and two small islands. the other was too big as in three schots. both it and the mainland one are large but lengthy for some reason. ive noticed through testing that land tends to be legthy but not always. either way. As for the oceans size its like 1k or more blocks apart. definnitely a journy and not a day trip, prefect for voyaging. have fun vanilla players!
Seed: 858540172