Subgrids added to a CRM form that is being rendered on a portal with a managed form will function according to the set of properties that are defined for that subgrid.  All options (excluding "Display Index" and Chart Options) will all be respected and alter the UI of the grid rendered in the portal. 

Sub-Grid Properties

Additionally the number of records per page is dictated by the Number of Rows assigned on the Formatting tab.

Note that the resultant grid will be read-only without further configuration.  In order to configure subgrids for use with Entity Forms or Web Forms, you will need to use Metadata records to define actions for the various operations you want made available on the form.  If you Enable Entity Permissions for your managed forms, these permissions will be respected as well.