Is it possible to have fields only controlled by specific users?

[Jeremy Malander]

Use case is for our developers to create project sizing (small, medium, large) after someone has submitted a request.  thanks!

[Sal]

Hey @Jeremy Malander,

I hope you're doing well, and welcome to the Atlassian Community!

I'm afraid not. Trello has no fine-tuned features, such as limiting who can use a custom field within your board. Currently, either the board member will have access to the board and the custom field to modify it, or they won't have access at all. 

Depending on what you're trying to achieve with Trello, custom fields may not be the best alternative.