Allocated Storage Space

The 100% you're seeing doesn't represent the actual amount of space you're using for storage. It represents the amount of space allocated for storage, i.e. your disk size. The df -h command will give you the accurate numbers on the amount of disk space that's actually being used.

To resize your Linode, just resize that disk to the size needed (as long as the actual used disk space will fit). Once that's done, you'll be able to resize your Linode