There will not be a specification which directly tells the smallest signal which can be amplified by an IA. There are specifications for the amount of noise which the IA creates at the inputs. Then, the signal is compared to the amount of noise and a "signal-to-noise" ratio can be found. The tricky part is the amount of noise depends on the frequency range required to see the signal. Generally, a wider frequency range for the signal means there will be more noise.
For example, if the signal is in the range of 1 Hz to 100 Hz, then the circuitry needs to be setup to amplifier this range of frequencies. Frequencies outside this range are eliminated using filter circuits and other methods. Once the frequency range is known, it is possible to estimate the amount of noise from information given in the data sheet.
Can you give more information on the signal you need to amplify? For example, is it a single frequency or a range of frequencies? For example, a human EKG signal is from approximately 0.05 Hz to 100 Hz. Then, we can start to look at the IA data sheet specifications and the other circuitry needed to reduce the frequency range and get rid of some IA noise.