How to add custom fields to an estimate like: SIZE? on the help section of quickbooks shows MANAGE CUSTOM FIELDS but that is not an option on my custom settings....

Let me share some information about adding custom fields to your estimates in QuickBooks Online (QBO), displayemporium-.

If you are using QBO Plus and Essentials versions, then we can add custom fields to your sales forms and purchase orders. To add custom fields to your estimates and name them as "Size", just follow the steps below: 

1. Go to the Gear icon on the top menu and select Custom fields.
2. Click Add field. If this is your first time creating a custom field, choose Add custom field.
3. Enter a name in the Name field.
4. Select the All Sales forms or Purchase Order checkbox or, select both.
5. To show the custom field on printed and delivered forms, turn on Print on form. Otherwise, it’ll only appear in QuickBooks Online.
6. Press Save.

For each custom field, you can see which forms it appears on. You also see the print icon if the custom field is visible to customers or vendors on a form. To manage all your custom fields, go to the Gear icon on the top menu, then select Custom fields. You can check out this article for more details: Add custom fields to sales forms and purchase orders in QuickBooks Online.

In addition, I've included this resource in case you want to personalize your sales forms so that they only display the information that's relevant to your company: Customize invoices, estimates, and sales receipts in QuickBooks Online.

The Comments section below is the best place to ask questions or post any other concerns about customizing fields in QBO if you have any. Don't hesitate to give me a shout and I'll be with you right away.