Multi-select picklist expression builder

Hello -

I have a muti-select picklist field in the Opportunity record type and I am trying to create a workflow that would need to check on how many values that field has and I am encountering two issues:

1- I cannot seem to be able to select the field from the Opportunity fields drop down in the expression builder. It simply does not show up. I checked other record types that also have multi-select picklists and the behavior seems to be the same.

2- Even if I am able to select the field, how do I find out how many values are on it using the expression builder functions.