Hello there, @ez7. 

Thanks for taking the time to reach out to us. Allow me to address your concern, so let me get this straightened out. 

As of now, setting up a unit of measurement for items in the purchase order is currently unavailable in QuickBooks Online. For the meantime, you can add a description for the unit of measurement when creating purchase order transaction. 

I can see how this feature would be helpful for you, I recommend sending feedback directly to our product developers for the future update. 

Here's how you can send feedback: 

1. Click the Gear Icon.
2. Select Feedback. 
3. Click Next and choose Feedback under Category option.
4. Select Save.

For additional reference, you can check this article: Enable and use Purchase Orders in QuickBooks Online.

Please know that you're always welcome to post here in QuickBooks Community if you have any other concerns. Have a good one.