Well, this is pretty tricky in any case, but I am not really sure what exactly you are after - a larger iris or a more open / more white showing eye itself.
DS uses thin walled cylinders, holding an eye of 24mm diameter by a spring with the newer versions having a sqare alignment bottom.
While this allows to move the iris somewhat easier by rotating the eyeball ( though sometimes the spring prefers unwanted positions) it also makes exchanging the eyes a bit more problematic, both against other DS eyes due to the whole cylinder needing exchange as well as towards non-DS eyes as you'd need to stuff something behind them to fill the compartment.
To actually get a larger iris you'd either have to take the DS mechanism apart, replace the original eyeball with a new one of the wanted iris size and then put the mechanism together again and back into the head or just replace their whole eye mechanism with an eye of your choice of about 24-25mm diameter and a lot of stuffing behind.
One way to make the eyeball optically larger( to show more white) without actually cutting into the material around the eye is to put some more material in the back, making the eye bulge slightly forward and thereby look bigger - but again, with the DS cylinders, despite their interesting design, this is more tricky.
Hope this helps a bit.