The reason behind this error is the Salesforce limit on the List object. The "Collection size exceeds maximum size of 1,000." is a standard Salesforce error. The List view has a maximum limit of 1000 records, which means that any List in Salesforce can fetch only 1000 records. In our scenario, when we click on the "Send SMS on Reports" tab, our code tries to upload all the reports in your org and that is where the limit is exceeded.
One temporary resolution on this would be to decrease the number of reports in your org and bring it down to <=1000.
Please note in latest version of "SMS On Reports" version 1.3-:
We can process
1)2k records per report.
2)Maximum reports supported is 9999 per org.
Please get in touch with SMS Magic Support team to get the link of SMS On Reports version 1.3.