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