Algorithm

Define a function isValidBST that checks if a given subtree rooted at a node is a valid BST by performing a recursive traversal with range constraints.
Initialize a variable maxSize to 0 to keep track of the maximum size of any valid BST subtree found. Recursively traverse each node of the given binary tree.
At each node, use the isValidBST function to check if the subtree rooted at that node is a valid BST. If it is, calculate the size of this subtree and update maxSize if this size is larger than the current maxSize.
After traversing all nodes, return maxSize, which will hold the size of the largest BST subtree found.