As far as the overall length of the slings, that's really up to you. Most of the slings I made for people are under 15', but occasionally I'll make one for someone who wants over 20'. IT all depends on the work you are doing.
For the eye size, it'll be about 20". Try it out knotted first, just to make sure. If it were me I'd use a 3 or 4 wrap klemheist. Err on the side of caution and make the eye bigger than you need. Make it too small and it's worthless!
First, what rope is that? Is it from the ebay guy?
The taper on the Samson splice IS fast. That's mostly because they are using a non-locking brummell, which you've replicated in your splice. They are looking for the compression of the outside strands to hold the splice together. If the taper started up further, to make a more gradual taper, you'd be cutting down on the holding power of the splice.
You have 2 options now, if you want a smoother splice. 1- You could bury MORE than recommended, which would allow you to spread your taper over a longer area but still keep the needed amount of non-tapered rope inside or 2- you can switch to a locked brummell and even shorten the tail a little AND spread the taper over a longer area.
With the locked brummell you lose a little bit of strength in the splice, but you gain a LOOOOOOOT of security.
I don't have any diagrams showing the locking brummell, (there are pics on the site, but they might be missing right now). I think I can explain it...
For your splice, you passed the working end through the standing end. This formed your eye. You passed the working end through the standing end AGAIN, then buried the little tail. Taper, bury, stitch and your splice is done. Your splice is unstitched right now. You should see that if you held the rope in a certain way and pulled on 1 side of the eye, you could take that splice apart in about 3 seconds.
For a locking brummell, you start the same. Pass the running end through the standing end to form the eye. Now you pass the STANDING end through the running end. Taper, bury, stitch and your splice is done. Except this time you'll see that this forms a "lock" where the tail CAN'T come out. More secure slightly less strong.
It's up to you which you prefer. They each have their pros and cons. The splice you have now is well done. However, don't use either without stitching that tail down!!!
The locking brummel shown on their site is tough to do in all but very loosely woven ropes. I doubt it'd be possible in the rope brendon is using. The good thing is that their technique is useful when you can't or don't want to pull the whole standing end through the running end. I use it when splicing beeline and HRC eye-eyes.
The locking brummell as I described is about as easy to form as the regular non-locking brummell.