i have a big problem in improvisation and the answers of the following questions will help me allot:

1. What does that mean "play over a chord progression"?

2.If i want to play a minor pentantonic scale over a chord progression must i play a box position or an expanded one?

3.Do i have to play the whole shape or a segment of it?
A chord progression is a sequence of chords that make up a song, or a section of a song. To play over, (or solo) over one you have to use scales that are relevant to them. You can play exclusively in the petatonic box but that will limit your soloing.
Basically a chord progression will be in a key. you can then improvise using that scale. for example if the chords were Am D C Em it would be in E minor and you could use an E minor scales to improvise or play over that chord progression.
the two replies are very helpful but i cant understand for a whole chord progression must play the scale or to change it depending on the key
IF you dont understand which scale to use over which set of chords, then welcome to understanding guitar isnt as esay as you may have thought. Every guitarist that doesnt know theory, will be in your place (pretty much). Not knowing which scales to use etc.

The simplest way is to use chords and the scales from songs you already learnt, mix them all up, and play with that.

The better way is to learn which scales go best where, which chords go best together etc. Which you will find how to do this, in the Beginner theory lessons on this site.

Hope this helped.