There aren’t any limits to the size of the schema that you can have.
Currently submitting a request to /admin/schema endpoint is recommended method. There is also updateGQLSchema mutation available of /admin endpoint. Could you please share some information on what way are you looking forward to?
Could you share some more details about the size of the schema and what is it that you are observing so that we can debug this? Does Dgraph crash or go OOM? How many resources are provisioned to Dgraph?