How can I delete all files in a folder with names that longer than X?

You can match filenames that consist of at least 10 characters using a simple glob expression ??????????*. So provided the number of files is not too long, you could use

echo rm -- ??????????*

For longer file lists, you could break the names up using xargs:

printf '%s\0' -- ??????????* | xargs -r0 echo rm

If you switch from bash to zsh, you could make use of glob qualifiers to test the length of each filename (passed via the REPLY variable). One advantage is that you can easily add other qualifiers like N (equivalent of the bash shell's nullglob) and . (which limits the results to plain files):

echo rm -- *(.Ne:'[[ $#REPLY -gt 9 ]]':)

or

print -rNC1 -- *(.Ne:'[[ $#REPLY -gt 9 ]]':) | xargs -r0 echo rm

To do the equivalent in bash would likely require a loop, like

for f in *; do
  [[ -f $f && $#f -gt 9 ]] && echo rm -- "$f"
done

Remove the echo once you are satisfied that the right files are being matched.