How to check in bash if file size is greater than N

You can do this using the find command and the [[ builtin,
wrapped in a one-liner like this:

[[ $(find /path/to/file -type f -size +51200c 2>/dev/null) ]] && echo true || echo false

• The find takes care of two things at once: checks if file exists and size is greater than 51200 bytes.
• We redirect stderr to /dev/null to hide the error message when the file doesn't exist.
• The output of find will be non-blank if the file matched both conditions, otherwise it will be blank.
• The [[ ... ]] evaluates to true or false if the output of find is non-blank or blank, respectively.

You can use this in if conditions, for example:

if [[ $(find /path/to/file -type f -size +51200c 2>/dev/null) ]]; then
    somecmd
fi