Thank you @ogra i was done the mistake, i have written only the size of the bin file.
i.e - $ll prime/u-boot-sunxi-with-spl.bin | cut -d ’ ’ -f5
# get the size
$ ls -l prime/u-boot-sunxi-with-spl.bin |cut -d’ ’ -f5
#add 1 block so we have enough space, divide by 512
$ echo $((419479/512))
get the actual minimal size
$ echo $((1073*512))
this help me to calculate the correct size as well as help me to understand the size parameter in gadget.yaml, that how the ubuntu-image uses the size while creating the partitions in the chunks of 512bytes.