Depends fully on your goal: Exploration could be taken first for several countries, but Economic could be interesting for a merchant republic, that will then use money to hire mercenaries and has not to expand too much due to size limitation.
Going tall isn't such a good plan for Merchant Republics in the long run. To get the most out of MR you need to build a substantial trade empire, and 'tall' trade empires don't really scale up in single player, as they suffer too much competition to maintain long routes. So you do want to expand a lot, it's just that almost all of the expansion will be in trade company land, where the province restriction doesn't matter (it'll all be 'territories' anyway, just that TC magic makes it zero autonomy).
If you want to go tall, I think it's better to be a non-merchant republic and really get into bed with the Burghers estate.