Please note; that just because the shoes do not fit does not give you a statutory right to a refund or replacement. Under the sale of goods act it is only if the goods are faulty or not of the quality / fit for their intended purpose (quality, not size), that the retailer is obliged to offer a refund / replacement.
Anything they offer you now is at the sole discretion of the retailer. However, if the shoes have not been worn or visibly marked on the soles, then I cannot see the fact you have removed the stickers, or the shoes are the wrong size affecting their decision to offer you a replacement / refund as a reasonable alternative.
Ask them in a polite manner and explain you made a simple mistake, you will have a greater chance of this problem being resolved to your advantage. !