Functions will not make your project file size noticeably smaller except possibly in extreme cases. They can save space in terms of number of events on your event sheet. If your 6 events are replaced with a function that also has 6 events, you will have the same number of events. However, if you use those 6 same events in 10 different places on your event sheet, you would only need to call the function once for each.
Note that additional actions do not constitute additional events - you can have as many actions per event as you want. The main benefit of functions is when making changes, you can make the change once and have it applied to all instances where the function is called, rather than having to change it in every instance separately.
As far as efficiency and system resources, functions have negligible impact in most cases.
To make your project smaller in terms of file size, the first thing to look at and reduce would be the number and resolution of your sprite textures and animations.