Hi Gareth,
I’ve had a look into this & the reason it’s appearing as 0.417ft3 in the takeoff is because 5" is 0.417ft. Since your Dialog Units are set to “inches” & Takeoff Units are set to “feet”, the value is being calculated. The value that needs top be entered into the Size field is the total amount of that object (e.g. if it was a 2ft x 2ft x 2ft cube, then 8ft should be entered).
Usually I’d say select the object & then look at the Volume listed in the Entity Info to get the value to enter into the Size field, however, in this case it appears it may not be the correct value (according to the measurements you provided & your screenshot - i.e. 255.62ft). Note: If the Volume field doesn’t appear in Entity Info, it means that SketchUp cannot calculate a value for the selected object as the geometry is the geometry is flat/2D or is broken in some way (aka the geometry isn’t manifold).
In this instance the value you’d need to enter into the Size field is 432000" or 250ft (depending if you want to enter inches or feet). Tip: Length fields will always calculate to the specified dialog units, so you can you enter any size & unit you like.
Another method to achieve your desired result, which should hopefully be quicker & easier for you, is to use the 
Custom Estimation Tool to get the volume quantity of the object/geometry. I responded to your other post with info on this which may be helpful (see link to post below).