Anything between unison and octaves are safe. Further up the fingerboard you can probably get a 10th. A good player will manage scales in 3rds, fourths, sixths, etc. Don't forget that no double stops are possible where the upper note is below D4, like C# above middle C is played on the G string. Also if one string is open you can get bigger intervals.
Edit: Forgot to mention that double stop harmonics are more difficult. Keep them to 4ths 5ths and 6ths, They need 4 fingers on the fingerboard. Have a listen to the 3rd movement of Paganini's Vn Concerto No 1 to find out what can be done but remember, there aren't many Paganinis about!