The overall shape is rather reminiscent of Great Britain actually. As for the rivers, well actually you may be OK. If your landmass is a couple of hundred miles across, like the UK, then most of the river islands are plausible sizes. You have an unusual number of them, but not impossible I think. I would reconsider the island at the junction halfway up the westernmost river, that does look odd, and the large island downstream in that same river should perhaps only have one channel around it active, and the other silted up.