First, check out FAQ: Is there a feature limit in ArcGIS Online? The limit generally applies as the maximum number of records that will be returned by the server. Since AGOL is a web service, retrieving too many records can cause the browser to have issues.
When I want to keep the feature small, I will divide the features into areas and create individual maps for each area. You will need to share the feature with a group, and if the feature is used in a Collector map, the map must also be shared with the group. You will need to allow Create, Delete, Query, Sync and Update operations for the group to be able to make additions and updates. If you are publishing from ArcMap you will need to select Capabilities >> Feature Access to set these options. These options can also be modified after publishing.
