How can I judge whether I would personally feel comfortable studying in a department with a large gender imbalance?

Do not judge by the gender ratio alone. First identify what might make you uncomfortable—such as isolation, speaking in class, group work, or practical activities. If possible, visit an open day or class, observe how students interact, and ask several current students about relationships and support. Check official information for consultation services and other support, then consider whether you could participate without undue strain and whom you could rely on if difficulties arose.