Show us some of your code. I think I am following you, but you didn't really give enough detail about your requirements or how you are implementing it as of now. Why is the maximum of 10 letters a problem for you? Give a bit more info and I know I can help.

Edit:

'get a 10-letter work from player 1, convert to uppercase
strWord = InputBox("Enter a 10-letter word:", _
"Hangman Game").ToUpper------------> That is correct.

'determine whether the word contains 10 letters
blnValidWord = True 'assume the word is valid----->That is fine.
If strWord.Length <> 10 Then

----->You are stating that you will only allow a number that is 10 characters long. What you want is a MAX of 10 characters. So heres what you should do:

If strWord.Length >10 Then

------> if strWord is greater than 10 characters, then blnValidWord is false

blnValidWord = False
Else

Dim intIndex As Integer ---> Dim all variables at the beginning of your subprocedure, not in the middle like this.

Do While intIndex < 10 And(--->"Also" should not be used) blnValidWord = True

If strWord.Substring(intIndex, 1) Like "[A-Z]" Then
blnValidWord = False -----> Wouldn't you want this to be true if the string contains characters [A-Z]?

End If
intIndex = intIndex + 1 ----> Could also use intIndex += 1
Loop
End If

'if the word is not valid, display a message
If blnValidWord = False Then
MessageBox.Show("10 letters are required.", _
"Hangman Game", MessageBoxButtons.OK, _
MessageBoxIcon.Information)
----> You could just use MsgBox("your text") in this instance.

As another tip, I would put the letters in an array while you are evaluating their validity if you plan to make a functional hangman game. Arrays would make this quite simple.
I did not point out all of the problems since it seems you are trying to learn VB, so let's see if you can finish it from here. If you have more questions, feel free to ask me.