Question 1: Is my innodb_log_buffer_size too small?

Answer:
Since MySQL 5.0 there is a status called Innodb_log_waits. This status shows the number of times that the log buffer was too small. A wait is required for it to be flushed before continuing.

SHOW GLOBAL STATUS 
LIKE 'innodb_log_waits';

+------------------+-------+
| Variable_name | Value |
+------------------+-------+
| Innodb_log_waits | 0 |
+------------------+-------+

If this value is 0 or near innodb_log_buffer_size is defined well. If it is high and continuously growing, increase it or reduce the size of your transactions.

Question 2: What would be a better value?
Answer: https://www.percona.com/blog/2008/11/21/how-to-calculate-a-good-innodb-log-file-size/