Not much of a definitive answer, but here are some prep works you should start doing:
If you did not obtain the Master program from this institute, gather their program information and understand it first. Key information include course offered (with syllabi if possible), degree audit (to figure out what is required and what is elective). This will give you a general idea of the students' level and capability.
Most theses are submitted to the school's own libraries or at least to their committee members. Check out a handful that are related to your field for an approximated scope.
If Master's defense presentations are open to public, go to a couple and learn about them.
Download the Master's degree program handbook and, if any, a list of instructions and obligations for committee members and skim them for useful information. These documents usually detail the length and specification of the project and thesis, as well as general procedure such as meeting frequency and responsibilities of all parties, etc. Be informed about these details helps a lot on budgeting time and effort.
You mentioned you're co-supervising, and I do hope each student also has at least one faculty member in the committee. Schedule an informal meeting with the faculty chair and ask for guidance. Unlike your supervisor, they have the obligation to care.
As for topics, by the time you have done the above you should have a good idea on the length, size, and depth. But some other tips here may be useful:
Dig out other Master or PhD topics that you might have given up.
Cast a look at your work, look for possible branches that you do not have time to explore. Try not to be overly possessive about these ideas, as you may not have time to actually explore them anyway.
One way is to think about your own thesis's Introduction. Are there any particular matters for which you wish there is a piece of evidence?
Start the habit of jotting down random ideas on handheld devices or a paper note pad.
Be extremely clear and open about the ownership of the ideas and deliveries before the student starts working on it.
Ideally (and I'm speaking from biomedical), the student should be technically capable to carry out 2/3 to 3/4 of the work. Try not to make them learn everything new or they can have integration anxiety and tremendous amount of self-doubt which leads to unrelenting reliance on you or worse, withdrawal.
Do know that (and I am sorry to be blunt) this supervising task is i) not a major determinant of your PhD graduation and ii) is but a small piece of evidence on mentoring that can get you hired. So don't invest excessive amount of time in it. And to achieve so, you need to first establish a sense of non-possessive attitude (the student is working on his/her Master, not your PhD). Second, the topic needs to be able to help you move along your study, but in the same time it should not be so crucial that if it failed your study will be held up. Third, the student should be able to do a good amount of it technically, and your job is to move them along by i) encouraging them to think and act critically and independently, ii) conducting meeting and communication regularly, iii) providing feedback and reasoned suggestions, and iv) demonstrating all the above three consistently.
