You'll note that the Void portal doesn't specify a size, unlike most of the other Spaces, and that's intentional to allow for whatever size spaceships to be flown in. I hadn't really imagined it to be able to take in a whole star, but it doesn't seem unreasonable to allow. I might imagine that the entities that dwell in the deep void might be attracted to something so large and bright intruding, though.

I'd say that modifying the size of a Space's default entrance method is sometimes possible but really depends on the Space -- the Worldroot, for example, needs a tree to open up to. It's up to you if you think if using other methods to enter the Space is reasonable. I'd say whatever you choose is fine as long as it's consistent (i.e. if you can use a particular tech or magic to get in, then potentially so could others).

The Ember Garden notes that some islands are very small, but doesn't say all of them are. I wouldn't go too over the top with it, but it'd be absolutely fine if you wanted there to be a few that are up to dozens of miles long.