Feature Extraction using pretrained CNN

Hi there.

With the help of a pre-trained CNN, can we extract a certain number of features from an image dataset?

For example by using SqueezeNet,extract only 100 features from image dataset.

My point is that while there is a layer with a defined number of units, how can we extract a desirable(100) number of features from that layer?

I know we can change the layer's detail easily, but the reassembling of the network may result in an error due to the empty weights of some layers.