Best way to fix the size of a sentence [Sentiment Analysis]
The easiest way is to average the word- embeddings. This works quite well. 
Another thing you can try is to represent each document as a bag of words - i.e. - to have a vector in the size of your vocabulary, where each element in the vector represents the number of times a certain word had been mentioned in your document (for example, the first element in the vector will represent how many times the word a was mentioned, and so on). 
Afterwords, to reduce the size of the vector you can use techniques like LDA, SVD, or autoencoders.