If you are just going to sew them together I suggest a narrow white border and then a wider multi-colour floral border, to include all, or most, of the colours in the pinwheels.
Is it big enough for what you want? It looks to me as though once sewn together it will be about
32" x 54" plus the borders, so maybe 40" x 62". If you add 1" sashing it might take it to 43" x 68".
Other ways of making it bigger (if that's what you want) would be putting the pinwheels on-point with plain white in between but that means doing something interesting in those white spaces, or making a few more pinwheels.