Hello....Take what does fit and enjoy the malls and American shopping prices and just above $1 = £1 so nearly half the price and at any rate you should save at least a third. Just remember that there will not be much in the way of heavier clothing in April in Florida. I think that Fort Lauderdale is better and has more to offer than Miami just in case you get an option....Heathe.
You are kidding - clothes are half price in the States. With the wonderful exchange rate it makes things even cheaper. Just take an empty suitcase and get yourself out to one of the outlet malls. You do not say where you are staying but my favourite one near Miami is Sawgrass Mills, just outside FLL about 45mins drive out of Miami.
If you like Guess and other designer brands go for it, bayside market place is good as is Lincoln mall area on south beach, but the best deals are the malls. We've spent time at Dolphin Mall nearer the airport than downtown and had some fantastic bargains. There are large sports and department store there and all the designer names. Adventura Mall and Coco Bay is also good for shopping, check out the mall websites. Downtown shops are a little tired and not so good though, depends where you are staying really.