Maybe this is useful to you:
A spherical layer + a Fractal Sum layer will get islands very similar to that.
…and if those results are pleasing, those two layer types have super simple math to generate them.
Edit: easier links:
http://mythruna.com/MapMaker/MapMaker-20120204-Windows.zip
http://mythruna.com/MapMaker/MapMaker-20120204-Linux.zip
http://mythruna.com/MapMaker/MapMaker-20120204-MacOSX.zip
Edit2: examples of what I mean after 10-15 seconds of clicking:
https://imgur.com/mq2fask.png
https://imgur.com/ZxfJSw2.png
I can provide instructions on how I got there… the parameters are a bit of an art. The terrain can also get much more realistic than that with different fractals, layering, etc…