Yeah, really it is all personal preference and depends heavily on your situation. If you have no trouble getting speed for wherever this rail is going to be located, you can afford to build bigger. I think anything shorter than 5 is too small. 5-10-5 would be good. But 6-8-6 or 8-8-8 or even 8-10-8 would be great. Whatever you think would be most fun and really evaluate your area and decide which measurements really fit your needs.
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