System.QueryException: Aggregate query has too many rows for direct assignment, use FOR loop

           This exception is sometimes thrown when accessing a large set of child records (200 or more) of a retrieved sObject inside the loop, or when getting the size of such a record set. For example, the query in the following SOQL for loop retrieves child contacts for a particular account. If this account contains more than 200 child contacts, the statements in the for loop cause an exception.

1. for (Account acct : [SELECT Id, Name, (SELECT Id, Name FROM Contacts) FROM Account LIMIT 100])
2. {
3.         // Assigning inner query values to List, checking list size , null conditions within the for loop leads the query exception
4.         List<Contact> contactList = acct.Contacts;  
5.         Integer count = acct.Contacts.size();
6.         if(acct.Contacts.size() > 0)
7.         {
8.        
9.         }
10.         if(acct.Contacts != null)
11.         {
12.        
13.         }
14. }

To avoid getting this exception, use a for loop to iterate over the child records, as follows.

1. for (Account acct : [SELECT Id, Name, (SELECT Id, Name FROM Contacts)
2.                     FROM Account LIMIT 100])
3. {                                      
4.     Integer count=0;
5.     for (Contact c : acct.Contacts)
6.     {
7.         count++;
8.     }
9. }

Reference: 

https://developer.salesforce.com/docs/atlas.en-us.apexcode.meta/apexcode/langCon_apex_loops_for_SOQL.htm