I am currently creating a song, and right now I'm doing the vocal line for a verse. Should vocal lines leap and take big intervals or only small leaps?
It depends who you're writing the song for. A professional opera singer? A community choir? Yourself?
Both very large and very small intervals can be difficult for untrained singers, as can accidentals. Songs with intervals of a fourth or less which stick to scale tones tend to be easier to learn.
As a very rough guide, if you have a look at the ABRSM sight singing syllabus on page 13 you'll see what intervals are to be expected at different grades. The intervals get wider and wider until grade 6, but then chromatic semitones come in, and finally diminished 7ths in grade 8. Note that this is for sight singing, the required pieces at each grade might be more challenging.
It's worth noting that if you have big leaps then the colour of the voice can change quite a bit, particularly if you're flipping in and out of falsetto, which can be very effective (e.g. Cee Lo Green - Forget You) but may not always be desirable.
At the end of the day if you like the sound of it and your singer(s) can cope with it then it's all good.
The conservative approach is best when jumping.
A good melody for voices will not be overly-jumpy. It will also have contrary motions in the melody line when it jumps, so, for instance, it is not good to move a step down after you have jumped down, it is also generally a good rule of thumb not to go a step up after you have jumped up.
Contrary movements after jumps (Step down after a jump up and step up after a jump down) is just a general rule that leads to good melody writing.
As for the intervals of leaps, you must remember that singers have no intonation given to them, some jumps are just simply very hard for them. Don't punish your singers at keep jumps to within a perfect fourth.
I know there may very well be cases where jumps of bigger intervals happen for perfectly valid reasons but let those cases be the exception, not the rule.
Leaps are EXCELLENT! There's a 'rule' however that a leap of an octave or more should be followed by a note that isn't even FURTHER away, and it's a generally good rule. But don't let it stop you writing a soaring phrase if that's what the song calls for.
A series of short steps moving up is generally followed by a leap down (or the other way round). Listen to "Blue Bossa", for example, G to G an octave apart and then down to F Eb D C. Check if your local library has Ernst Toch's The Shaping Forces in Music.
Variety is the spice of life. A leap can add excitement and drama to a melody line, but you don't want to overdo the drama, do you? And remember that some leaps are easier for the singer than others. The occasional octave is fine, provided you stay in range; a major seventh might be harder for an untrained singer to hit accurately.
I don't think there's a rigid rule - sometimes large leaps work, sometimes they don't.
However, here's something to try if you're writing tunes - experimentally change one of your descending small intervals (for instance descending from C to B or C to A) into a large ascending interval (C up to B or C up to A). Used sparingly this can transform a dull stepwise line into something with more spice and drama - don't overdo it though.