There is no exact limit for that. Sometimes, in the model file, there is limited transitor size for accurate spice model. You can check your model file
are you sure the maximum is 20um? probably that is digital process. please consult you foundry wheter they have others suitable process for analog circuit.
Using one of the attached architectures will help u to obtain good swing at the output node although and you'll need to use small W's and L's for switching transistors.
Attached in also is the paper from which i captured these two configurations, I recommend that you read the whole paper, it's really useful.
For me, i recommend the second configuration (figure b), due to its low complexity, although it have some draw-backs which tried to be solved in figure a, i don't recommend figure a. Read details about them in the paper.