Question: Let's say I create a repository, add **x** files to it and commit. Say the size is **a** Mb after the initial commit. 

* Is there any way to estimate how large the repository is going to be in one years time?
* If the lines of code has increased by 10%, will the repository have grown accordingly?
* How does number of commits, branches, tags etc. factor into the repository size?
* Will 10000 commits the same year make the repository grow (noticeably) more than say 1000 commits?
* Maybe my question is wrongly phrased?

Answer: 
Take a look at [GitBenchmarks](https://git.wiki.kernel.org/index.php/GitBenchmarks) page on Git wiki, the section "Repository size benchmarks" and "Other benchmarks and references" (taking into account *when* the benchmark was made, and what *versions* it uses), in particular the entry at the end page:

* [DVCS Round-up: One System to Rule Them All? -- Part 3](http://ldn.linuxfoundation.org/article/dvcs-round-one-system-rule-them-all-part-3) by Robert Fendt on Linux Developer Network, from 27-01-2009, contains results of two synthetic benchmarks testing how a system acts under stress (number of commits in repository, or number of files comitted).

The test system was a VM running Ubuntu 8.10, and the software versions used were SVK 2.0.2 (last is 2.2.3), darcs 2.1.0 (last is 2.4.4), monotone 0.42 (last is 0.48), Bazaar 1.10 (last is **2.2.1**), Mercurial 1.1.2 (last is 1.6.4), and Git 1.6.1 (last is 1.7.3).
If you're worried about size mushroomin, go and clone some online projects and examine the size of their repositories. There are plenty of large projects to choose from with branches commits, etc, etc. My experience is that git & mercurial and pretty good about keeping size down, the size is a reflection more of the files that you put into them (and their size) rather than overhead.