How about this?

!k:: ;Hotkey is Alt+K
Loop 1500
{
Send +{Right}
}
Send {Esc}
Return

The problem with this script is that it will also count spaces and tags as characters.

I'm sure experienced coders can come up with something more elegant.

Thank you Nora,

It works with plain text, but unfortunately not with tables. Probably I should describe why I need this. For example, I have a 2-column export docx that I have to divide in equal parts between several translators. Now I just select and copy some portion of text (roughly) from the export file and paste it into a new document to know how many chars are in there. Ideally, I want a script to count and select cells based on the not-greater-than-1500-chars condition. In case anybody knows how to do that, I would really appreciate your sharing this sacred knowledge. So far I will use the script from Nora, it is definitely better then what I have now (=nothing). Thank you again 

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Even though I admit the work should be paid and it is fair, but I am not sure if this is a good idea of doing business here on an SDL resource. If you are serious, email me.

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krupa adrian, I need the script to select cells containing not more than 150/1500/15000/whatever characters, rather than split them. I don't want you to spend your precious time on what is not needed. Don't bother to make the "merge" code or whatever you call it. Just forget it. Thank you