I became really annoyed while trying to create a multipart mob and the hitboxes were not visible. It turns out that minecraft does not actually render the hitboxes when pressing F3+B but just a box with a size of width*height*width centered on the mobs position.

 

I modified the code slightly and thought it might be a good idea to incorporate this into forge.

 

Here is the changed function in net.minecraft.client.renderer.entity.RenderManager.java

 

private void func_85094_b(Entity par1Entity, double par2, double par4, double par6, float par8, float par9)
    {
        //GL11.glDepthMask(false);
        GL11.glDisable(GL11.GL_TEXTURE_2D);
        GL11.glDisable(GL11.GL_LIGHTING);
        GL11.glDisable(GL11.GL_CULL_FACE);
        GL11.glDisable(GL11.GL_BLEND);
        GL11.glPushMatrix();
        Tessellator var10 = Tessellator.instance;
        var10.startDrawingQuads();
        var10.setColorRGBA(255, 255, 255, 32);
        
        var10.addVertex(par1Entity.boundingBox.minX, par1Entity.boundingBox.minY, par1Entity.boundingBox.minZ);
        var10.addVertex(par1Entity.boundingBox.minX, par1Entity.boundingBox.maxY, par1Entity.boundingBox.minZ);
        var10.addVertex(par1Entity.boundingBox.minX, par1Entity.boundingBox.maxY, par1Entity.boundingBox.maxZ);
        var10.addVertex(par1Entity.boundingBox.minX, par1Entity.boundingBox.minY, par1Entity.boundingBox.maxZ);
                   
        var10.addVertex(par1Entity.boundingBox.minX, par1Entity.boundingBox.minY, par1Entity.boundingBox.minZ);
        var10.addVertex(par1Entity.boundingBox.maxX, par1Entity.boundingBox.minY, par1Entity.boundingBox.minZ);
        var10.addVertex(par1Entity.boundingBox.maxX, par1Entity.boundingBox.maxY, par1Entity.boundingBox.minZ);
        var10.addVertex(par1Entity.boundingBox.minX, par1Entity.boundingBox.maxY, par1Entity.boundingBox.minZ);
                       
        var10.addVertex(par1Entity.boundingBox.maxX, par1Entity.boundingBox.minY, par1Entity.boundingBox.minZ);
        var10.addVertex(par1Entity.boundingBox.maxX, par1Entity.boundingBox.maxY, par1Entity.boundingBox.minZ);
        var10.addVertex(par1Entity.boundingBox.maxX, par1Entity.boundingBox.maxY, par1Entity.boundingBox.maxZ);
        var10.addVertex(par1Entity.boundingBox.maxX, par1Entity.boundingBox.minY, par1Entity.boundingBox.maxZ);
                     
        var10.addVertex(par1Entity.boundingBox.minX, par1Entity.boundingBox.minY, par1Entity.boundingBox.maxZ);
        var10.addVertex(par1Entity.boundingBox.maxX, par1Entity.boundingBox.minY, par1Entity.boundingBox.maxZ);
        var10.addVertex(par1Entity.boundingBox.maxX, par1Entity.boundingBox.maxY, par1Entity.boundingBox.maxZ);
        var10.addVertex(par1Entity.boundingBox.minX, par1Entity.boundingBox.maxY, par1Entity.boundingBox.maxZ);
        
        Entity[] parts = par1Entity.getParts();
        if(parts != null)
        {
        for(Entity part : parts)
        {
            var10.addVertex(part.boundingBox.minX, part.boundingBox.minY, part.boundingBox.minZ);
            var10.addVertex(part.boundingBox.minX, part.boundingBox.maxY, part.boundingBox.minZ);
            var10.addVertex(part.boundingBox.minX, part.boundingBox.maxY, part.boundingBox.maxZ);
            var10.addVertex(part.boundingBox.minX, part.boundingBox.minY, part.boundingBox.maxZ);
                 
            var10.addVertex(part.boundingBox.minX, part.boundingBox.minY, part.boundingBox.minZ);
            var10.addVertex(part.boundingBox.maxX, part.boundingBox.minY, part.boundingBox.minZ);
            var10.addVertex(part.boundingBox.maxX, part.boundingBox.maxY, part.boundingBox.minZ);
            var10.addVertex(part.boundingBox.minX, part.boundingBox.maxY, part.boundingBox.minZ);
               
            var10.addVertex(part.boundingBox.maxX, part.boundingBox.minY, part.boundingBox.minZ);
            var10.addVertex(part.boundingBox.maxX, part.boundingBox.maxY, part.boundingBox.minZ);
            var10.addVertex(part.boundingBox.maxX, part.boundingBox.maxY, part.boundingBox.maxZ);
            var10.addVertex(part.boundingBox.maxX, part.boundingBox.minY, part.boundingBox.maxZ);
                    
            var10.addVertex(part.boundingBox.minX, part.boundingBox.minY, part.boundingBox.maxZ);
            var10.addVertex(part.boundingBox.maxX, part.boundingBox.minY, part.boundingBox.maxZ);
            var10.addVertex(part.boundingBox.maxX, part.boundingBox.maxY, part.boundingBox.maxZ);
            var10.addVertex(part.boundingBox.minX, part.boundingBox.maxY, part.boundingBox.maxZ);
        }
        }
        
        GL11.glTranslatef((float)(par2-par1Entity.posX), (float)(par4-par1Entity.posY), (float)(par6-par1Entity.posZ));
        var10.draw();
        GL11.glPopMatrix();
        GL11.glEnable(GL11.GL_TEXTURE_2D);
        GL11.glEnable(GL11.GL_LIGHTING);
        GL11.glEnable(GL11.GL_CULL_FACE);
        GL11.glDisable(GL11.GL_BLEND);
        //GL11.glDepthMask(true);
    }

did u tried setSize in the entity class constructor???