Quedar vs. caber
When talking about clothes fitting in Spanish, the words quedar and caber have distinct uses. The word quedar focuses on how clothing looks or feels on your body, such as being too loose, too tight, or looking stylish. On the other hand, the word caber focuses strictly on physical space, meaning whether your body can physically enter or squeeze inside the garment at all.
Quedar
The word quedar is used to express how an item of clothing fits or looks on a person, usually describing the style, length, or tailoring by using adjectives like big, small, tight, or good.
La camisa me queda muy apretada de los hombros.
(The shirt fits me very tight in the shoulders.)
Estos pantalones te quedan perfectos.
(These pants fit you perfectly.)
El vestido le queda grande y necesita arreglarlo pronto.
(The dress fits her big and she needs to alter it soon.)
No sé si me queda bien esta talla de zapatos.
(I do not know if this shoe size fits me well.)
La chaqueta nos queda un poco corta de mangas.
(The jacket fits us a bit short in the sleeves.)
A mi hijo ya no le quedan sus camisetas del año pasado.
(My son's t-shirts from last year no longer fit him.)
Caber
The word caber is used to express physical capacity or volume, specifically indicating whether a person can physically get into a piece of clothing or if the garment has enough literal room to contain them.
He subido de peso y ya no quepo en estos vaqueros.
(I have gained weight and I no longer fit in these jeans.)
El abrigo es de un niño pequeño por lo que no me cabe.
(The coat belongs to a small child so it does not fit me.)
Intenta ponerte las botas para ver si te caben.
(Try putting the boots on to see if they fit you.)
Tienes el pie muy ancho y no vas a caber en esos zapatos.
(You have a very wide foot and you are not going to fit in those shoes.)
El sombrero no le cabe en la cabeza por todo el pelo que tiene.
(The hat does not fit on his head because of all the hair he has.)
Engordé tanto durante las vacaciones que ahora no quepo en mi traje.
(I gained so much weight during the holidays that now I do not fit in my suit.)
Summary
In short, you should use quedar to describe how a piece of clothing suits you, fits your proportions, or drapes on your body. Conversely, you must use caber when talking about the literal physical space and whether you can successfully get inside the garment without it being physically impossible.