Excel workbooks can maintain a relatively small file size even when they have thousands of rows, columns, or cells with long formulas. The amount of data that Rowan can process at any single time — called "context window" — is vast, but has limits nonetheless.
If the workbook that you're working on is too large for Rowan, you'll see a warning sign in the bottom left of the chat box. You can click on this warning to deselect sheets that are irrelevant to your task, getting the spreadsheet back within Rowan's context window.
Top tip: Even when you're within the context window, it can sometimes help to still deselect irrelevant sheets to help focus Rowan's attention on the task at hand.