The Gen 4 21 has the same frame as the old "SF" model, the backstraps allow you to make it bigger if needed.

Yes the Glock 21 is slim grip. The width is 32.50 mm / 1.27 in. LENGTH: 209 mm / 8.22 in. WIDTH: 32.50 mm / 1.27 in. LENGTH BETWEEN SIGHTS: 172 mm / 6.77 in. HEIGHT: 139 mm / 5.47 in. BARREL HEIGHT: 32 mm / 1.26 in. BARREL LENGTH: 117 mm / 4.60 in. budsgunshop.com/catalog/product_info.php/products_id/411548960/New+Glock+21+FDE+Gen+4+45acp The Full dimensions can be found here - https://thegunparlor.com/product/fde-glock-g21

Wesley, The Modular Back Strap design on the G21 Gen4 lets you instantly customize its grip to adapt to an individual shooter's hand size. budsgunshop.com/catalog/product_info.php/products_id/411548960/New+Glock+21+FDE+Gen+4+45acp

LENGTH:
209 mm / 8.22 in.
WIDTH:
32.50 mm / 1.27 in.
LENGTH BETWEEN SIGHTS:
172 mm / 6.77 in.
HEIGHT:
139 mm / 5.47 in.
BARREL HEIGHT:
32 mm / 1.26 in.
BARREL LENGTH:
117 mm / 4.60 in.

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Hi Weslie, Yes the Glock 21 is slim grip. The width is 32.50 mm / 1.27 in. budsgunshop.com/catalog/product_info.php/products_id/411548960/New+Glock+21+FDE+Gen+4+45acp