How to increase the gain of Operational transconductance amplifier by changing the sizes of transistors.
Your voltage gain will be 

$$
\frac {V_{net3}} {V_{INP}-V_{INN}} = \frac {g_{m2}} {g_{ds2}+g_{ds5}}
$$

Assuming the NET3 is your output. Since for a given current density gm is proportional to the DC current, (\$ gm = \frac {2 I_{DC}} {V_{GS}-V_{th}}\$) and in saturation gds is also proportional to the DC current (\$ g_{ds}=\lambda*I_{DC}\$), the intrinsic gain of the transistors of your technology limits your gain. You can not change the intrinsic gain, but you can use an additional stage or stages to increase the gain. For example cascoding, where the second stage is a common gate stage.