Import troubleshooting: Document workaround for a too large repo

.markdownlint.yml

@@ -49,6 +49,7 @@
     "Geo",
     "Git LFS",
     "git-annex",
+    "git-sizer",
     "Git",
     "Gitaly",
     "GitHub",

doc/topics/git/git_rebase.md

@@ -174,18 +174,18 @@ the operation you want to perform in each commit. To do so, you need to edit
 the commits in your terminal's text editor.
 
 For example, if you're using [Vim](https://www.vim.org/) as the text editor in
-a macOS's `ZSH` shell, and you want to **squash** all the three commits
+a macOS's `ZSH` shell, and you want to `squash` or `fixup` all the three commits
 (join them into one):
 
 1. Press <!-- vale gitlab.FirstPerson = NO --> <kbd>i</kbd> <!-- vale gitlab.FirstPerson = YES -->
    on your keyboard to switch to Vim's editing mode.
 1. Navigate with your keyboard arrows to edit the **second** commit keyword
-   from `pick` to `squash` (or `s`). Do the same to the **third** commit.
+   from `pick` to `squash` or `fixup` (or `s` or `f`). Do the same to the **third** commit.
    The first commit should be left **unchanged** (`pick`) as we want to squash
    the second and third into the first.
 1. Press <kbd>Escape</kbd> to leave the editing mode.
 1. Type `:wq` to "write" (save) and "quit".
-1. Git outputs the commit message so you have a chance to edit it:
+1. When squashing, Git outputs the commit message so you have a chance to edit it:
    - All lines starting with `#` are ignored and not included in the commit
    message. Everything else is included.
    - To leave it as it is, type `:wq`. To edit the commit message: switch to the

doc/user/project/repository/reducing_the_repo_size_using_git.md

@@ -13,6 +13,8 @@ Git repositories become larger over time. When large files are added to a Git re
 - They take up a large amount of storage space on the server.
 - Git repository storage limits [can be reached](#storage-limits).
 
+Such problems can be detected with [git-sizer](https://github.com/github/git-sizer#getting-started).
+
 Rewriting a repository can remove unwanted history to make the repository smaller.
 We **recommend [`git filter-repo`](https://github.com/newren/git-filter-repo/blob/main/README.md)**
 over [`git filter-branch`](https://git-scm.com/docs/git-filter-branch) and

doc/user/project/settings/import_export.md

@@ -210,3 +210,59 @@ To help avoid abuse, by default, users are rate limited to:
 | Import           | 6 projects per minute                |
 
 GitLab.com may have [different settings](../../gitlab_com/index.md#importexport) from the defaults.
+
+## Troubleshooting
+
+### Import workaround for large repositories 
+
+[Maximum import size limitations](#importing-the-project)
+can prevent an import from being successful.
+If changing the import limits is not possible,
+the following local workflow can be used to temporarily
+reduce the repository size for another import attempt.
+
+1. Create a temporary working directory from the export: 
+
+    ```shell
+    EXPORT=<filename-without-extension>
+
+    mkdir "$EXPORT"
+    tar -xf "$EXPORT".tar.gz --directory="$EXPORT"/
+    cd "$EXPORT"/
+    git clone project.bundle
+
+    # Prevent interference with recreating an importable file later
+    mv project.bundle ../"$EXPORT"-original.bundle
+    mv ../"$EXPORT".tar.gz ../"$EXPORT"-original.tar.gz
+    ```
+
+1. To reduce the repository size,
+   [identify and remove large files](../repository/reducing_the_repo_size_using_git.md)
+   or [interactively rebase and fixup](../../../topics/git/git_rebase.md#interactive-rebase)
+   to reduce the number of commits.
+
+    ```shell
+    # Reduce the .git/objects/pack/ file size
+    cd project
+    git reflog expire --expire=now --all
+    git gc --prune=now --aggressive
+
+    # Prepare recreating an importable file 
+    git bundle create ../project.bundle <default-branch-name>
+    cd ..
+    mv project/ ../"$EXPORT"-project
+    cd ..
+
+    # Recreate an importable file 
+    tar -czf "$EXPORT"-smaller.tar.gz --directory="$EXPORT"/ .
+    ```
+
+1. Import this new, smaller file into GitLab.
+1. In a full clone of the original repository,
+   use `git remote set-url origin <new-url> && git push --force --all`
+   to complete the import.
+1. Update the imported repository's
+   [branch protection rules](../protected_branches.md) and
+   its [default branch](../repository/branches/default.md), and
+   delete the temporary, `smaller-…` branch, and
+   the local, temporary data.