Panicdog,

I've not used Instagram but some web searches found the 640x640 pixels is the typical image size. Did you create a project with the dimensions? eg File >> Settings >> Movie, [E] key shortcut, or from the startup screen? There are lots of tutorials for other editors which might guide your choice for export format. 

It would help if you could outline what steps you have tried that are giving you a hard time.

Regards,

Carl