Creating check of file getting to certain size

Tags: #<Tag:0x00007f84b208b4a0> #<Tag:0x00007f84b208b360>

Hi,
I’m looking for a way to check if a certain file in MSSQL Server goes over a certain size (9GB) and alerts. This is to warn of SQL Express db files getting too big for the server to handle. I have SNMP monitoring on the servers in question. I was thinking I could set up an alert notification rule using storage.storage_size. Does this use kb? I’m not having much luck finding documentation on setting these up from scratch. My scripting knowledge is basic. Any pointers would be much appreciated.