Strategies for self-learners to transition into working on larger projects
You have set a difficult task. The reason is that there are two dimensions here, not one. The first is that larger projects are usually built by teams and not by individuals. Certainly if you are interested in preparing for employment, then teamwork is essential. The second dimension, independent of the first is that larger projects are normally built to the vision and specification of someone other than its builders. 

This leaves four cells in a grid. You probably already have experience building alone and to your own vision. This is the easy case. The hardest is building in a team to someone else's specification. You can get there immediately but it is very difficult to transition that way. Instead you can first move through one of the other cells. 


You can find a team to work with you on some project of your own vision. You can, perhaps, find a description of such a project online or in a course description. You can also just try to replicate some software that you normally use that seems big to you. Build a spreadsheet program, for example. The hard part, however, is to find the team to work with. Note that the project need not be terribly large, but you will learn important large-project skills in any case. 
An alternate way is to move into the other cell. Build something yourself, but with another person specifying the program. This will be a lot harder than what you usually do, but is an essential step. One way to find the problem is to find the client. It could be a teacher with a need. It could be a local social service with a need. It could be anyone with a need. But you need to give complete control over what is to be built over to the client, while you are creative only on the how to do it. Again, the size of the project is less important than that you are learning how to work with a client. 


Once you have some experience in one of the above, you will be ready to move to the externally specified-team built cell. But the experience in the simpler case may give you some insight into how to find the team and the client. Scaling at this point will be more reasonable than you think. 

Good Luck. 



I should not that even in formal education, when a project is specified by a professor, it isn't quite the same as working for a client. The typical university project normally grants the student a large part of the design space. A true client will not. Normally the professor won't ask you to change direction in the middle of a project. A client might. A professor might grade you. A client might just reject your work or ask you to do it over. A university project is normally thought out completely in advance by the professor. A client may make it up as you go along.