Typically, for an 11 year old player we would recommend getting a 1B mitt in a 12.00 inch pattern. A mitt such as the 2017 Rawlings Gamer Youth Pro Taper 12" First Base Mitt: GFM16B would thus be an ideal fit. Whether to use a 12.00 inch pattern or a 12.50 like the Rawlings Sandlot Series 12.5" First Base Mitt: SFM18 will come down to personal preference as far as what size feels comfortable to the player. With that being said, the Rawlings Sandlot Series 12.5" First Base Mitt: SFM18 will have more of an adult fit with relationship to the wrist opening. This may benefit a bigger 11 year old player compared to the youth pro taper fit of the Rawlings Gamer.
At your player's age, it would be hard to suggest a better first base mitt than the 2017 Rawlings Gamer Youth Pro Taper 12" First Base Mitt: GFM16B. It features a snug fit for a young player, and the overall quality of the glove would be hard to beat in another youth model.
Unfortunately I don't believe Rawlings offers this glove in a customizable format. You could try contacting them directly for more information.