Deinterlace by dropping even lines from command line
ffmpeg -i input -vf field,scale=123:456 output will do the trick, where 123:456 is your x:y frame dimensions for the output. The field filter just chucks away the fields it doesn't use. Documention on the field filter here. Note that you'll need extra stuff in the command to get good results - e.g. you'll probably want to specify codec details and so on.