I wrote I want a white small star because I figured the Welsh adjectives where in the same order but afterwards, i.e. with small closest to the noun and white the furthest away.
I think small comes before white in English. It's more common, anyway, and sounds a bit odd otherwise. I'm not sure there'd be any reason to put it that way round unless you were specifying a white variety of something pre-established called a 'small star' :/
Adjectivial order is a funny thing! Speakers of English don't often recognise the underlying orders that they're taught.
"I want a small, white star. Small is the main adjective while white is only a modifier.
There are a handful of adjectives that have separate masculine and feminine forms; "white" is one of them, being gwyn for masculine nouns and gwen for feminine ones.
Then remember that an adjective after a feminine singular noun will mutate softly, and you get seren wen for "white star".