Here's the relevant part of the official docs.
Min and max:
GPU memory in megabytes, sets the memory split between the CPU and GPU; the CPU gets the remaining memory. The minimum value is 16; the technical maximum value 944 [for the RPi 4]. The default value is 64, values above 512 will not provide increased performance and should not be used.
You shouldn't need to set it beyond the default value in the plain Raspbian /boot/config.txt. Max value is 128M.
Setting it beyond 128M can leave your Raspberry Pi 4B unbootable.
GPU memory on a RPi4B isn't used in the same way as on earlier Raspberries.