Should I create multiple ISession instances in my client application?

Normally you should use one ISession instance per application. You should share that instance between classes within your application.

How can I enable logging in the driver?

The driver allows you to plug in any ILoggerProvider implementation, like NLog and Serilog implementations.

You should set the provider before initializing the cluster, using the Diagnostics class:

// Use the provider you prefer, in this case NLog
ILoggerProvider provider = new NLogLoggerProvider();
// Add it before initializing the Cluster

You can configure the log levels you want to output using the provider API.

Alternatively, if you don’t want to use a ILoggerProvider implementation, the driver can expose log events using the .NET Tracing API.

// Specify the minimum trace level you want to see
Cassandra.Diagnostics.CassandraTraceSwitch.Level = TraceLevel.Info;
// Add a standard .NET trace listener
Trace.Listeners.Add(new ConsoleTraceListener());

It depends on the size of the requests and the number of tables affected by the BATCH. Large batches can cause a lot of stress on the coordinator. Consider that Cassandra batches are not suitable for bulk loading, there are dedicated tools for that. Batches allow you to group related updates in a single request, so keep the BATCH size small (in the order of tens).

Starting from Cassandra version 2.0.8, the node issues a warning if the batch size is greater than 5K.

What is the best way to retrieve multiple rows that contain large-sized blobs?

You can decrease the number of rows retrieved per page. By using the SetPageSize() method on a statement, you instruct the driver to retrieve fewer rows per request (the default is 5000).