"The buttons are small."

Translation: Die Knöpfe sind klein.

[SundarGm]

Buttons are non living so shouldn't it be kurz instead of klein?

[Langohr_]

„kurz“ means “short”, not “small”.

[TomHockenh]

Why is it Die here but Der when youre trying to say the button?

[quis_lib_duo]

(der) Knopf is a masculine noun.

Here, you have the word in plural nominative for which the article is die.

[Emmanuelle675133]

Plural words always go to "Die"
The gender of the word Knopf is Der.
So der Knopf - Die Knöpfen
The button - the buttons.

[biertopf]

"der" is not the gender, but the definite article used for a masculine (that's the gender) noun in nominative case singular.

[Ladybug746639]

Der Knopf (Einzahl)
Die Knöpfe (Mehrzahl)

[saurabh402629]

I related Der Knopf with the knob and it is easy to remember this way.