I suppose the question must be read as asking for a ballpark figure of what would be a reasonable slit separation to use to observe an interference pattern at reasonable angles. You will need to do something like the following:
- Decide on what angle you would like to observe the first intensity maximum at
- Find a formula relating the angle of the first maximum to slit separation (I bet that's easily found on wikipedia or in most textbooks on the subject)
- Find out what the wavelength of "red light" is
- Use the information you have from above to calculate the slit separation.
When doing the above, you should feel free to use approximations and round off numbers you look up. Good luck!