What degree would be useful for working as secretary for a CEO or big company ?
It is not very likely that you will become a CEO’s secretary right off the bat, no matter what you do - it is more likely that you will be the secretary for a VP or Director, who is then promoted to executive VP and eventually selected to be CEO.
If your goal is to be on the admin side of things and work as a secretary or administrative assistant, then a bachelor’s degree may not make sense unless finance is not an issue. There are associate’s degrees and/or certificates that can prepare you for those types of roles and may prove just as useful as a bachelor’s degree but without the high price tag of a four-year university. I’d look more broadly than bachelor’s degrees.
With an accounting degree, secretarial/administrative work would generally be considered underemployment, since you would be qualified to work in higher-level roles as soon as you graduate (think of it like a lawyer working as a paralegal or an electrical engineer working as an electrician). Underemployment isn’t a bad thing if you want to be doing that work, but it just means that you can achieve the same goals without as much post-secondary education.
You could study anything, really, since there is no specialty for the position you desire. Like chrisw said, you would primarily need to focus your time on internships, networking, and working your way up to that position, which could take some time. What’s important is that whatever you choose to study provides you with the skills you need for that job, such as time management, strong communication skills, negotiation, etc.
Having some knowledge of the industry (e.g., if you work for a tech company, understanding the basics of what they do) and some business management skills in general could help. Even if you’re not the one making the decisions or doing the technical work, understanding what’s going on around you within the organization can help you to thrive in your support role and may assist in your ability to move up the ladder. Of course, you don’t need a degree for any of that. You could learn on your own, on the job, through internships, and other avenues.
When people in office support jobs do have a bachelor’s degree, it is most likely to be in business (http://www.census.gov/dataviz/visualizations/stem/stem-html/ - choose Non-STEM tab & put your cursor on ‘Office support’). People with degrees in Business Management are sometimes seen working as administrative assistants and the like (http://www.payscale.com/research/US/Degree=Bachelor_of_Science_(BS_%2f_BSc)%2c_Business_Management/Salary), but it does not appear that people with accounting degrees work to any significant extent as mere administrative assistants (http://www.payscale.com/research/US/Degree=Bachelor_of_Accountancy_(BAcc)/Salary).