It is possible
No.
If she has had her period before, then yes
It is a bit borderline but not entirely impossible.
Yes, it is possible (depending on how developed the girl is, of course), for an 11-year-old boy to get a 13-year-old girl pregnant.
No.
Yes, if a girl has a period and the boy has started to ejaculate pregnancy is most certainly possible.
A girl can get pregnant as soon as she starts having monthlys, so if a girl gets them that early, then yes she could get pregnant. The earliest I've ever heard of though was an 11 year old who had a c section.
If the eleven year old is producing sperm and the fifteen year old girl is producing eggs (has started her periods/menstrual cycle), then yes, the girl can become pregnant. The girl does not even need to have had her first period. Her first egg is produced before her first period. However, in many countries where both are underage it would be illegal to do so.
Weight and height have nothing to do with pregnancy. If the girl has a period she can get pregnant.
personality
11 years old....