This three cases are a bit special than general cases.
1. Talking about if someone is a little fat, I agree with Vanessa. But I would like to add something: if for example I've gained some weight, I probably will say "Estoy un poco gorda". But imagine that you know me in that moment, for you, my normal weight is the one that you see in me in this moment, so if you describe me, you will probably say "Ella es un poco gorda".
2. Talking about having an angry personality, there's not an adjective in Spanish that can define the personality of a person as in "ella es simpática" or "él es alegre" that we can use with the verb "ser". Ways to say approximately the same: ella está siempre enfadada, ella siempre está enojada, él está siempre de mal humor, él es un amargado.
3. Talking about if someone is tall, if we describe an adult, we say always "él es alto". Only if the adult uses high heels, we would say "qué alto/alta estás con esos tacones" or something like that. If we talk about a kid and we want to emphasize that we see a change in the height, we will say "el niño está muy alto", "tu niña está alta". But if we describe the natural height of this kid, we say "él es alto".
In general, if we talk about a person defining him or her, we use "ser", and if we talk about a person describing his or her state, we use "estar". But with adjectives that describe states like contento o enfadado, we can say "siempre está...." if we want to express a usual state in a person.