Increasing nextcloud disk size

Nextcloud version (eg, 12.0.2): 12.0.5
Operating system and version (eg, Ubuntu 17.04): Ubuntu 16.04

Quick question, I’m running a nextcloud instance on a server with a 25gb disk, if I simply upgrade to the next size server will nextcloud automatically resize?

That is not really a nextcloud question. But to answer it. Are you talking about a virtual machine? If so, well it depends on the virtualisation type. Do you know what type it is? KVM or VPS for example? KVM could be a problem because if you change the size of the virtual disk it is not automatically changed within the VM.