File name beginning with - (dash)

Use ./ before filename:

for i in *; do stat -c "%s %n" "./$i"; done

Or use -- to indicate the end of options for stat:

for i in *; do stat -c "%s %n" -- "$i"; done

Though that one will still fail for a file called - (will report information for the file open on stdin instead of the - file in the current directory).

Add -- to mark the end of the options to stat:

for i in *; do stat -c "%s %n" -- "$i"; done