I think word-word.com is more search engine friendly than wordword.com because search engine robot recognize the words better with a separative symbol , but existing a dash in your domain name decrease domain cost!
I'm not sure about domain but RFC 2616 - Hypertext Transfer Protocol -- HTTP/1.1 does not specify any limits on URL length.
Web browser do have their limits, however.
Internet Explorer, the most common web browser, has a limit of 2,048 characters in a URL. This is documented in Microsoft Knowledge Base Article 208427.
Well, in my opinion, wordword.com is always better, but I have gotten some good results from the SE's with word-word domains too though, so it's hard to tell!
As for domain length, it doesn't matter for SEO purposes, but I like a domain that is less than 16 characters.
While having keywords in a domain is good (search engine and branding wise), however have long domain name or with a dash in middle takes away the branding aspect, since it is harder to remember.
The shorter the better for brand recall. But it would really depend on the availability of your desired domain name. If your first choice is already taken (in all tld's) you may want to add another relevant word to it. Not quite sure if a dash affects your SEO though, but personally I would go for the choice without it. Just choose the a domain which is most relevant to your niche and oe that would be easier for SEO
domain name length
Just choose the a domain which is most relevant to your niche and oe that would be easier for SEO 
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