May I ask for time to think before answering a big request from my spouse?

Yes, and it is even good; asking for a little time before answering a big request leads to a better decision than a hasty answer. Say kindly that you would like to think about it first, rather than refusing or ignoring it. Set a time when you will give your answer so your spouse is not left waiting. Use that time for salat al-istikharah, weighing your capacity, and discussing if needed. Do not use ‘later’ as a way of avoiding it forever. Your spouse will also trust an answer that comes from consideration rather than pressure. An answer thought through calmly is more respected and rarely regretted. See communication & emotions.