How to sort files with a certain permission by size?

In your standard bash shell, you can do that with the find command:

find . -perm 0644 -printf '%s %p\n'| sort -nr

This finds all files and directories with permissions 0644 and prints the results formatted using the -printf action. %s represents the file size and %p the file path. \n represents a new line. The output is piped into sort, set to sort numerically (-n) and in reverse order (-r).

Using the Z shell (zsh) with its glob qualifiers:

print -rC1 *(.DNf:u=6,go=4:oL)

where the meanings are

• . matches plain files only
• D sets the GLOB_DOTS option for the current pattern
• N sets the NULL_GLOB option for the current pattern
• fspec matches files with access rights matching spec
• oL orders the results by length (size in bytes); use OL to reverse the order