Add the "unity-base-field__aligned" uss class to it. (you can access it through ObjectField.alignedFieldUssClassName or BaseField<Object>.alignedFieldUssClassName)
public abstract class BaseField<TValueType>
{
// ...
public static readonly string ussClassName = "unity-base-field";
public static readonly string alignedFieldUssClassName = BaseField<TValueType>.ussClassName + "__aligned";
// ...
}
Any BaseField<T> with that style applied, that is located inside the Inspector will align its label.
Also if under an element (or self) with "unity-inspector-element" class and another parent with "unity-inspector-main-container" class, the last one defines the available width.