A sizing system for an article of footwear includes an insert attachable to the upper of the article of footwear. The insert is positioned within a pocket formed between the lining of the upper and the exterior shell of the upper. When attached to the upper, the insert forms a part of the exterior surface of the article of footwear. When the insert is removed, part of the pocket forms a portion of the exterior surface of the article of footwear or a flap is extended over the pocket to form a portion of the exterior surface of the article of footwear. The insert may be secured within the pocket using a mechanical fastener.