Hi,

Here is the answer of your questions.

1) what should be the max, appropriate length of a domain name?
The maximum length of domain name is 63 characters
(with letters, numbers, dashes)
And other 4 character for domain extensions like [.com, .net] etc…

2) what is better word-word.com or wordword.com?
as seo experience and want advices from all of you.
i suggest you wordword.com is better than word-word.com
why ?
B'coz in domain name and special characters like "-" is not for good this
when SE crawled it will count as negative point to your site.

No it won't

__________________
And how can we win, when fools can be kings

Is it not effect to our site ?

Nope,

use hyphens, don't use hyphens it will make precious little difference

Too many and it will look like crap for the real users, but SEs won't care.

__________________
And how can we win, when fools can be kings

it should not be morethan 65 char including spaces..

wordword def. better than word-word. As for length, always remember the shorter the better!

Spaces are NOT allowed in hostnames.

__________________
And how can we win, when fools can be kings

pick a short and easy to remember domain name..
using hyphens or underscore has no advantage over URLs w/o word separator.. they are treated the same by SEs

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No they aren't.

only hyphens are used as word seperators.

__________________
And how can we win, when fools can be kings

The shorter the domain names length the better, both for the search engines and especially for people. You know, people can easily remember the great website they have recently visited by the name only if it is relative short in length but sometimes, it seems that all the good names are gone and so in such cases, the dash marks might be considered.