Re: what happen to the size of molecules duing the phase change?Explain.

I will not do all your homework for you but generally a phase change is a change 
in the physical state of a material - a change between the states of matter 
(solid, liquid, or gas).  The size of the molecule would be expected to change 
little or not at all, however, the interaction between adjacent molecules would 
change dramatically (strength of bonds, distance between adjacent molecules, 
etc.).  

It sounds like the intermolecular distance is what your question is getting at. 
In terms of kinetic theory, the velocity of the molecules is also much changed 
when they change their phase (physical state).  This is usually, but not always, 
due to a change in temperature. You can learn more about this behavior in 
freshman college chemistry texts, or upper division physical chemistry texts.  
If you are not in college, your chemistry teacher can direct you to appropriate 
books.