India has clear natural borders all around not completely unpassable, but very solid. By way of land I think the best they could do is conquering Burma and holding Afghanistan and parts of Iran, while gobbling up Tibet and parts of Turkesta. Most of all, it depemnds on their capacity of diffusing a cohesive imperial culture. such as India seldom had.
By way of sea, things are far more interesting. Historically, South India had close ties with all Southeastern Asian countries, which Hindicized (sp?) significantly, aprt from Sinicized Vietnam. A colonial thalassocracy could so include, like colonies or subject kingdoms, all of Indonesia (with further exploration to the Pacific), Malaya, Kampuchea, southern Vietnam (Hindu Cham),
and in the Indian Ocean Madagascar, Reunion and Mauritius, Znzibar, maybe the Cape colony, Oman, Yemen.
It all depends on how much the Indians could reach in inner unity, tolerance, scientific development etc. Realistically, very few. With some luck... well, who knows?