The ornamental design for a beanie with a sizeadjustment fastener, as shown and described.
FIG. 1 is a front perspective view of a beanie with size adjustment fastener showing my new design;
FIG. 2 is a right side elevation view thereof;
FIG. 3 is a rear elevation view of my new design in which the strap is shown in a closed position; and,
FIG. 4 is a left side elevation view thereof.
The partial human figure depicted by broken lines assists in showing how the claimed design may appear when worn and forms no part of the claimed design.