The maximum number of characters for a text message sent from Skype is 160*. If you type a longer message it will be broken down into two or more text messages (depending on the length of the message) and you will be charged accordingly.
Skype's text message window tells you how many characters you have remaining as you type the message, and the cost of the text.
*There are two formats for text messages – GSM character set and Unicode character set. You can send 160 characters with GSM-supported characters, but if you enter a character that is not supported, the format of the message will change to Unicode. The maximum number of characters for a Unicode text message in Skype is 70.