I'm guessing that you saw my old thread about installing a 5" Garmin on a Pro.Fit bracket: http://www.clublexus.com/forums/ls40...lan-nokia.html
Go as big as you like. People have even installed iPads on brackets next to their LS400 HVAC controls. The 5" Garmin was plenty big for me.
My opinion is that the LS400 head unit location is too low for a nav unit but plenty of people have installed a double-DIN with nav in that location. I later had a double-DIN Kenwood without nav installed to replace my Nakamichi head unit but I continued my Garmin. I love the way that the nicer Garmins interface with phones.via the Garmin Smarphone Link app - available on Google Play for Android phones and I think there is now an iPhone version.