I'm reading a paper on counting cells, humans, etc. Is there an off-the-shelf library for Python/Theano or TensorFlow I could use?
I think the OP was asking for a specific library for Theano or TensorFlow that provides off-the-shelf functionality for object identification.
I would look first into
scikit image, an image processing library for Python in the Scipy/Numpy stack. It has all sorts of edge detectors, blob detectors, shape identifications, color separations... Look at their gallery of examples.
Btw, counting humans in a crowd may not be so trivial to automate...
Yes, there are libraries for TensorFlow and Theano.
In addition, I would also like to suggest you to have a look at this question regarding open source Python deep learning libraries.
To see what you can do considering the size(area) of objects, the android app PHOTO COUNT do the job using opencv 3.1 library.
Not sure I understand the question... TensorFlow is OpenSourced, here is a tutorial
To import the TensorFlow library, all you need to do is:
import tensorflow as tf