battery size

Hey Y'all, Working on my first RC build :applause: Progress: motor, props, tx/rx, and batteries are all in, waiting on an ESC from china. I got bored this weekend and bought 2 sheets of foamboard, and was surprised to get an FT FLYER and a NUTBALL out of 2 sheets!! BUT, I have a problem...

I'm building a foam replica of a kadet LT 40 I owned a few years back. I know it's a trainer plane, but it was quite a fun plane inverted three feet of the flying field. Anyway I have found enough info to get it looking like it's older brother but I don't know what size motor/battery to use...