I don’t know for what exact task you want to extract a feature vector for but, in theory, you can obviously use a pre-trained resnet to extract a feature vector of any size from an image. Keep in mind that such pre-trained resnets were specifically trained for image classification and so they will extract features with the sole purpose of image classification. However, you can easily circumvent this by doing transfer learning and re-training the last few fully connected layers according to your task.