多大 is about the size (how big, how old)
多少 is about the quantity (how many, how much)
(1) 他的年齡有多大？
(2) 他多少歲？
In (1), the question is asked from the point of view of the accumulation.
In (2), it is asked from the point of view of the precise number solely.
Could these two terms be used interchangeably? No, because even if they do are used in the same linguistic context, the meaning of the sentences won't be the same.
(3) 他給你提供的幫助有多大？
(4) 他給你提供的幫助有多少？
In (3), it is about the importance of the help (thus it is more subjective).
In (4), it is solely about the number (from an already expected amount).