My guess would be as large as possible. depending on any limit of the TV. Video tends to create large files, and you need to decide whether you intend having one film/programme per memory stick, or a number of films held on the one.
You can record 4.7 GB in 120 minutes on a Standard DVD disc from the TV so maybe that will give you some idea when selecting your memory stick?
Just a thought!
If it is the Tesco one (which I have) you can plug in a USB hard drive into it and record load of stuff. Although I have an 8GB USB Stick which keeps me going for a few hours.
I have to say the sound is pretty poor (I have a 19" TV). I don't know if the larger TV's have better speakers. The picture is pretty good although sometimes the picture does not sync very well with the sound. I have to change the channel away and then back and it seems to re-sync.
To improve the sound. I bought USB powered speakers HP ones from the internet. When I am not recording (which is most of the time) I have these speakers plugged in which has a much better sound.