Please solve in below way:

Press space key when u-boot in the countdown, stop at the u-boot command line and input two lines of commands:

MYD_AM335X# set nand_img_siz '0x500000'
MYD_AM335X# saveenv

Other commands for reference:

MYD_AM335X# print nand_boot
MYD_AM335X# print nand_img_siz