Hi Strugglingwithicloud,
To make sure we're on the same page, you are attempting to sync a file over to your iPhone via iTunes, but are prompted there isn't enough space on the iPhone for the file?
If so, does your iPhone show ample storage under Settings > General > iPhone Storage specifically? Also, to confirm are you attempting to sync the file using the steps outlined in the following article?
If your iPhone does not show enough free space under Settings, take a look here at tips to manage your storage:
Finally, is it only this specific file you're unable to transfer? Or are you unable to sync any other file? What is the file type you're attempting to sync.
Feel free to reply to this post with the requested details or if you have any further questions.
Cheers!