Let M be a subst of {1,2,...,2006} with the following property: For any three elements x,y and z (x<y<z) of M, x+y does not divide z. Determine the largest possible size of M. Justify your claim.
1. **Understanding the Problem:**
   We need to find the largest subset \( M \) of \(\{1, 2, \ldots, 2006\}\) such that for any three elements \( x, y, z \) in \( M \) with \( x < y < z \), the sum \( x + y \) does not divide \( z \).

2. **Rephrasing the Condition:**
   The condition \( x + y \) does not divide \( z \) can be simplified. Since \( x < y < z \), \( x + y \neq z \) is a necessary condition. However, we need to ensure that \( x + y \) does not divide \( z \) for any \( z \) in \( M \).

3. **Constructing the Subset \( M \):**
   Let's consider the largest element in \( M \) to be \( n \). We will analyze the cases when \( n \) is even and when \( n \) is odd.

4. **Case 1: \( n \) is even:**
   - Consider the pairs \(\{1, n-1\}, \{2, n-2\}, \ldots, \{\frac{n}{2}-1, \frac{n}{2}+1\}, \{\frac{n}{2}\}\).
   - Each pair \(\{a, b\}\) is such that \( a + b = n \). If we take more than one element from any pair, their sum will be \( n \), which is not allowed.
   - Therefore, we can take at most one element from each pair.
   - The number of such pairs is \(\frac{n}{2}\), and we can take one element from each pair plus the middle element if \( n \) is even.
   - Thus, \(|M| \leq \left\lfloor \frac{n}{2} \right\rfloor + 1\).

5. **Case 2: \( n \) is odd:**
   - Consider the pairs \(\{1, n-1\}, \{2, n-2\}, \ldots, \left\{\left\lfloor \frac{n}{2} \right\rfloor - 1, \left\lfloor \frac{n}{2} \right\rfloor + 1\right\}\).
   - Similar to the even case, we can take at most one element from each pair.
   - The number of such pairs is \(\left\lfloor \frac{n}{2} \right\rfloor\), and we can take one element from each pair.
   - Thus, \(|M| \leq \left\lfloor \frac{n}{2} \right\rfloor + 1\).

6. **Applying to \( n = 2006 \):**
   - Since \( 2006 \) is even, we have \(|M| \leq \left\lfloor \frac{2006}{2} \right\rfloor + 1 = 1003 + 1 = 1004\).

7. **Constructing a Valid Subset \( M \) with 1004 Elements:**
   - Consider the subset \(\{1003, 1004, \ldots, 2006\}\).
   - This subset has 1004 elements.
   - For any three elements \( x, y, z \) in this subset, \( x + y \) will be greater than \( z \) because the smallest sum \( 1003 + 1004 = 2007 \) is greater than the largest element 2006.
   - Therefore, this subset satisfies the condition.

Conclusion:
The largest possible size of \( M \) is 1004.

The final answer is \(\boxed{1004}\).