@C2i the below questions are just to help the post.
Will you be installing an actually server in the cabinet? Most servers requires a 1meter deep cabinet. Unless it's a micro server.
1 meter deep cabinets start mostly from 27u and up.
A small cabinet is normally considered as 9u, 12u and 15u. They start from 550mm deep.
9u or 12u should be sufficient for basic network equipment or micro server.