The entire $25,000 is allowed, and you can't choose to take a smaller amount.
You will then have negative income on your tax return. That is called a "Net Operating Loss" (NOL), which is a separate carry-forward. TurboTax should notify you of that, and give you an option to "waive the carryback" and carry it to next year.
Other options to "waiving the carryback" would be to amend your 2015 (and possibly your 2016) tax returns to use the NOL. You could also use Form 1045 to use it on this year's tax return, but TurboTax does NOT support that form
From what I understand, TurboTax also does NOT calculate the actual amount of the NOL that can be carried forward (or back). You would need to manually calculate that, or you may want to go to a tax professional.
https://www.irs.gov/pub/irs-pdf/p536.pdf
