Long as you cut the tube to the right size it'll be fine. The bigger you are the moulding agent you need. As for the silicone there's loads! We had a bit left over. If its one with the vibrator and you are concerned about the amount you'll need you can buy a larger unit from the clone-a-willy site.
Hope you are having fun, we did it a few years ago and loved it, but our first go didn't work. My bf is too big and I had to bin the first load and buy 2 more. It worked with 2. Best thing ever :-) btw, it helped when he put a cockring on.
We are thinking of getting a clone a willy kit but we have just ordered a clone a pussy kit first to see how long you have to work with the contense and how difficult it is
It took us 2 goes to make 1. The first one we took too long and the powder mix set way to fast for me to insert into it. The second go was a few weeks later as we didn't buy extra powder with the first purchase. Something to consider for those trying it for the first time. It is fun to do, but the my wife finds the silicon clone of me to be way too firm for her to enjoy. She was hoping it would have been a bit more softer and have some flex so that she could use it without discomfort, we did get the vibrating version so I don't know if that takes out the flex. Not sure if the firmness is our fault in the mixing stage or just the quality of the material. So my clone is tucked away in a drawer with all the other toys we tend to not use anymore. Not sure why I keep them, maybe in the hope that one day they will get used again. Who knows!