Depends on both the rating of the li-po pack (should match/be just under peak discharge capacity) and whether you want some more protection for your speed controllers (then you'd go for a smaller fuse, but run the risk of blowing this just through heavy loads/stalled motors).
Are you still using these http://cgi.ebay.co.uk/18-5v-2200MAH-45C ... 231019e093 as I noticed you'd posted earlier? - why two packs, surely simpler to use one?