Although the sentence is in past tense, because the clause still holds true, 'is' should be used instead. Meaning, the person is still short-tempered.
If you use 'was', it would mean that the person is no longer short-tempered. This is not what you meant to say.
A) If I were to fight someone, I would choose to fight someone who is small-sized.
Note: 'were' and 'would' show that the sentence is in past tense, but because the person is still small-sized we use 'is'.
B) Jane, who is the director of Taste Inc, was my girlfriend in college.
Note: Jane is still the director so we use 'is' but no longer my girlfriend so we use 'was'.
Feel free to ask if you don't understand anything.
On a side note, it should be 'any problems' instead of 'any problem'. We always use the plural form of the word after 'any'. Example: Do you have any cigarettes?