The underlying ideal is that every line should be an attractive and singable melody. The harmony should sound like it came naturally out of the melodic lines. The melodic lines should not sound like random strings of notes dictated by the chord changes. Having big leaps, or many leaps, or repeated leaps in the same direction is a problem if it's a symptom of this "random string of notes" syndrome.
If you're not sure whether something is OK, sing it yourself (transposing to fit your own vocal range). Try to sing it from the score without touching the keyboard. If you have trouble singing it, then other people will too. If you think it's awkward or boring to sing, then so will they. If you think an inner part lacks interest when it stands on its own, then tenors and altos will probably agree with you.
Intervals are not hard or easy to sing in and of themselves. It depends on context. For example, there's nothing hard in general about singing an ascending major second -- but it could be very hard if you gave someone a part that was trotting along happily in the key of C and then made a surprising, unprepared step from B to C#.
On the other hand, a leap of a major 7th could be very easy to sing if it was (in C) C B C'. It's a little formula used in cadences, and people will recognize it instantly and be able to sing it fine.
There is a distinction between a vocal style and an instrumental style. For instance, the first Bach cello suite starts with G D B A | B D B D, etc. It's fun to try to sing this, and it can sound nice if you can find the right key to make it work for your voice, but it's fundamentally an instrumental line, not a vocal one. It's natural for the bow to saw back and forth across the strings. It's not natural for the human voice to do this.