Cannot "Purge files from repository history" : error 413

I followed this guide to reduce the size of my free repository on
For starters, I tried to purge a large file from repository history.
However, at the step 9, it failed to push the changes:
I suppose that the reason is that the size of the upload is too high?
Anyway, I’m wondering how can I resolve the size quotas of the free account, without actually buying new storage, which I would need only to reduce the size of the repository?

Another thing is that tommorow I will not be able to write anything to the repository.
Again - how will I be able to reduce the size of the repository, without buying extra storage that I will not actually use?