Thank you Miles, saves me having to say it!
Cutting tenons vertically, with whatever jig/machine always has the restriction of height. Either working height above the floor (for jigs like the Rat, Leigh, or my own vertical router tenon jig) or ceiling height (for any table saw tenon jig, such as the Delta or even the best in the world, The Ultimate Tablesw Tenon Jig). It is one reason why I got rid of my Leigh jig, I didn't like having to stand on a box and work half way up the wall (plus it was a pain to set up. Results were perfect, though, provided I didn't cock up, which was, sadly, all too easy to do).
For long lengths, cutting the tenons horizontally is a much safer, more convenient and generally easier option.
If you have a bandsaw, invest in a top quality rip blade. I've just installed one of Ian John's TuffCut blades and it is the best rip blade I've ever used. With my fine adjust rip fence I can take off 0.5mm without the blade being deflected, something my previous blade wouldn't do, even when brand new. With just a spacer for tenon width (don't forget to take the kerf into account) you will have perfect tenons in no time at all, without buying any jig whatsoever and having nothing taking up valuable workshop space.
If you don't have a BS, then do as Miles suggests and make my Large Tenon Jig. It's not fast, but it is accurate and enables me to cut large tenons - very large if necessary -with a normal router. It'll take you an hour to make.
BTW The 5" x 1/2" limit quoted for the Leigh will be the size of the tenon, not the stock. 5" wide, 1/2" thick, and as long as the cutter you use. It means you can't make a 5/8 tenon with it.
HTH
Steve
PS I wonder if I shall make 4K posts today?