Digital BW, The Print

I'm putting together a portfolio to show potential customers.  Can the 
photos be 4x6 or is 8x10 the standard?  Is matte preferred over glossy 
paper?  My portfolio will consist of b&w, color, and sepia.  Should 
b&w and sepia be printed on matte paper?  Does it present better on 
that type of paper?  What about the color photos?  Will they look more 
professional on matte or glossy paper?  Can any type of photograph be 
printed using matte paper?