convert multidimensional feature vector to fixed size

I have a multidimensional feature vector for the N image, which varies in length.

I had to add NaN for images with a small vector length to be stored in the matrix!

I have several key points for the N image . And that's my feature length for different images is different.

Does the bag of feature method apply here? How is this at all?

How do I convert a feature set to a fixed feature to get the vector size for all images (rows) fixed?

Is there a way?