That is how big could the Brazilian empire plausible get:

What about more lowly populated plains and jungle east of the Andes? Could the western border of Brazil be farther west in Paraguay, Bolivia, eastern Peru, etc.?

There is no stimulus to do that apart from "Hey let's seize some land to make Brazil bigger". Brazil has more than enought agricultural and pastoral land, making those plains useless. Brazil also would need to keep a garrison there and try to populate it and to integrate the local minorities, it would be a drain for the economy.
 
Honestly the best bet if you really want bigger size is Brazil to have kept some of Portugal's colonies after independence. This is not as absurd as it sounds because the African ones were tied a lot more closely to it than Portugal but this alone requires some divergences and will cause a lot of changes after.

Aside from that Uruguay, The Guyanas and pieces of Argentina and Paraguay are the ones that make the most sense. Perhaps a few islands here and there too I suppose if you really want more.
 
There actually were plans in Angola to follow Brazil, but we have also to remember that Pedro declared independence from his father, the king of Portugal. Ultimately, to maintain parts of Africa into the Empire would be the worst economically speaking, as it would give a boost to slavery. The only way it can actually be good to Brazil is if this direct rule over Africa makes the Brits really pissed to the point that they intervene directly in Brazil, prompting social change and the end of slavery, somwhat like Post-Crimean Russia.

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IMHO Uruguayan independence was the worst thing that happened to Brazil during the 19th century internationally already during the 1820's. IOTL later in the 19th century the main geopolitical goal of the Empire was its loose indirect rule over the Platine region, this 'not so ambitious' goal clearly reflects the Empire's ambitions internally: Brazil was just a sleepy and lazy giant trying to maintain a fragile status quo at all costs.
 
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