I think you should focus on what interests you. The scale/complexity is somewhat immaterial.
My first ever PHP project back in 2001 was a guestbook. It read/wrote to files on my server as it's "database". This was pretty simple.. one message per file, saved in a directory.
My second PHP project was a forum like vBulletin. Vastly more complex, used MySQL as the backend, multi-user accounts, etc. The code, in today's world was horribly insecure and wide open to SQL injection and XSS attacks.
The guestbook in reality would probably have been a better codebase to showcase as there was less to do and it was likely more secure (there was no database, so that removes SQL injection, for example).
Personally, what I look for when hiring people is how they perceive the problem and how they go about a solution. Keep the code clean, simple, maintainable. I'd sooner see a few simple projects of really decent quality than something that looked like it was thrown against a wall to see what sticks.
Complexity will come with experience.. start "small" and focus on the quality of that.
An example... we recently(ish) hired a person that was essentially a junior dev. He showed decent code, on par with what was to be expected with his experience, however, it was the answers to questions that ultimately scored him the position. We weren't looking for a junior per-se, but his answers clearly showed that he understood the problems, how he would go about dealing with those problems, his thought processes and his passion (this is a key quality, far more than technical ability.. I can teach you to write decent code/tests.. I can't teach you to be enthusiastic about what you do)... so I personally really wouldn't worry about complexity if you're looking to start your engineering career.. focus on clean, simple architecture that a company can see would be easy for a team to maintain in a few years time.. that offers them value and therefore, makes you more of an attractive hire, IMO.