Your current account balance is $X smaller/less than is required
I have read that when we are talking about amount we say small amount. So in my case what word would be right to use?
It is possible, at least hypothetically, for a "balance" to be negative. A large negative balance is certainly not smaller than a small positive balance, but it is less, by the accepted meaning of the word.
To say that one amount is smaller than another is, strictly speaking, to preclude the possibility of its being negative, or otherwise to assume that one is speaking of non-negative amounts. The word "balance" implies an amount that could, at least hypothetically, be positive, negative, or zero---as balances tend to be explicitly signed quantities, debit or credit in some sense---and therefore to me it does not seem appropriate to say that one "balance" is smaller than another unless one is speaking in terms of absolute value, but even this would not usually be inferred on its own without explicitly making the situation clear.
For example, if A's balance is -$5.00 and B's balance is -$3.00, then both of the following would be true:
In the original example, without further context, I would only use less than.
The phrase less than is a mathematical phrase that requires an expression on both the left and right side.
The word smaller denotes size and could be used as an indicator.
Numeric values can represent size, but money is not a measurement of size. Money is best used to measure budgets, balances and other financial information.
A numeric example that works well with small as a logical comparison.
Where as, using less than is not as informative.
Mathematically that is not true, 12 is greater than 3. So small works better since it refers to size.