[U-Boot] Learn size of file in ram from bootloader

I have loaded a file in ram memory in location 0x80800000. I use a custom command in the bootloader named update as shown below.

update rootfs ram 0x80800000 0x02853000

The value 0x02853000 corresponds to the size of a file which is not stable. How can I take the size of the file with a function (sizeof(file) ?) in uboot?

Thanks