Unable to allocate enough memory to render current frame

Are you rendering out through the Render Queue? Or Media Encoder?

How do I change the size to 50 or 75%?

You'd have to check with the authors of the plugins you're using to see if you could temporarily transfer / use them on another machine. It depends on their individual licencing arrangements.

One final thing I would suggest: Do you have to render at that size?  Could you render at 50% or 75% size and scale up the resulting video once output?

However I may be able to open after effects on another computer (a Mac) so is there a way to transfer the aep files to that computer?

The composition itself is very simple- it is just an edit with zoom in's and out's. The only effects applied to the composition are MBL, twixtor, and sapphire plugins. 

Unfortunately, with the laptop you have - a Macbook Air - I don't believe you can upgrade the RAM, as it's soldered onto the motherboard. So, rather than buying a new computer... please let us know:

Why do you need output this big - what are you doing with it?
Does your composition / layers have effects applied, and if so what are they?

If we have this information we *might* be able to offer some help in rendering out this composition with the computer resources you have. Otherwise you'll have to find someone who can render this for you on a machine with more RAM.

How do I get more RAM?

No, upgrade your hardware: get more RAM.

Do you mean update my macOS? Because I currently have catalina. 

Can you upgrade the computer so that it meets the minimum requirements?