Normally users are not required to estimate the deal size while creating a deal. In the early stages of a deal, there may not be enough information to make an accurate estimate, and forcing users to make an estimate can pollute your data.
However, in some businesses, it can be possible to estimate a deal size from the very beginning. This is especially true for big-ticket items like real estate, motor vehicles, etc. where the customer is interested in a single specific item with a relatively fixed price. The seller has a reasonably good idea what the buyer will pay if he agrees to buy. If you are in such a business, you may want to force your users to enter an estimate while creating a deal.
Go to Control panel >Deal Settings > Settings. Enable expected amount is required and Save.