Saddles don't particularly come in set sizes for the different uke scales (soprano, concert, tenor etc.), it's more just variation between different kinds of bridge from different makers.
Slimming the bridge saddle down is no big deal, just rub it on abrasive paper that is clamped to a flat surface.
It's the piezo element that may be trickier. If it is a cheap pickup I'd just try sanding away some material from each side and hope I didn't wreck the element. Rather that than start screwing with the bridge itself, unless you're experienced.
If it's a cheap element then the worst that can happen is that you wreck it and are out a few $. If it's an expensive pickup then I'd a) ask the retailer if I can return it for exchange or refund, b) take the whole lot to a shop and ask an experienced instrument tech to fit it for me, or b) Sell the pickup or put it away in a drawer and find one whose dimensions will fit the slot.