You could try adding the tech requirements of the 6th hull size twice, to see if this will fool the AI into using the 6th  hull size.  
 
  
Name                          := Escort
Short Name                    := Escort
Description                   := 
Code                          := ES
Primary Bitmap Name           := Escort
Alternate Bitmap Name         := Escort
Vehicle Type                  := Ship
Tonnage                       := 150
Cost Minerals                 := 150
Cost Organics                 := 0
Cost Radioactives             := 0
Engines Per Move              := 1
Number of Tech Req            := 2
Tech Area Req 1               := Ship Construction
Tech Level Req 1              := 1
Tech Area Req 2               := Ship Construction
Tech Level Req 2              := 1