I did not play around the HEC for Splunk cloud. Per my research the HEC is enabled by default for your Splunk Cloud environment with a 1 MB size limit on the maximum content length.
For further information about the cloud limits and constraints, check the link below:
https://docs.splunk.com/Documentation/SplunkCloud/latest/Service/SplunkCloudservice#Service_limits_a...
If you need to increase the capacity
https://docs.splunk.com/Documentation/SplunkCloud/latest/Service/SplunkCloudservice#Subscription_exp...
For Splunk Enterprise there are two parameter that you have to play around. They are "queueSize" and "persistentQueueSize" that you have to setup at inputs.conf under http stanza like this one: [http://yourtokenname]
queueSize The maximum size of the input queue in memory. The value of this parameter is in the form [KB|MB|GB]. The default value is 500KB.
persistentQueueSize = [KB|MB|GB|TB]
* Maximum size of the persistent queue file.
* Persistent queues can help prevent loss of transient data. For information on
persistent queues and how the 'queueSize' and 'persistentQueueSize' settings
interact, search the online documentation for "persistent queues"..
* If you set this to a value other than 0, then 'persistentQueueSize' must
be larger than either the in-memory queue size (as defined by the
'queueSize' setting in inputs.conf or 'maxSize' settings in [queue] stanzas
in server.conf).
* Default: 0 (no persistent queue).
https://docs.splunk.com/Documentation/Splunk/8.0.0/Data/UseHECusingconffiles#Per-token_settings
