what is the largest possible Netherlands on Continental Europe?
The answer to this question is rather hard, partly because it is such a broad question. The best way to answer it, is to look at what oppertunities arose for the Netherlands to expand.
Lets first ignore Bakker-Schut, the megalomanic idea of post WWII Netherlands to annex part of Germany:
http://en.wikipedia.org/wiki/Dutch_...erritory_after_World_War_II#Bakker_Schut_Plan
Yeah, that's not going to happen. Possibly on a much smaller scale, like the OTL annexations (which were later returned to Germany). Anyway, I can't see the allies agreeing to this, as long as they are willing to use Germany as a buffer against the Soviet Union. Without the Soviet Union it might be possible, but on a much smaller scale.
Next megalomanic idea to shoot down is king Willem I's idea to not only add Belgium to the Netherlands, but also a large part of the Rhineland (the area between the rivers Moselle and Rhine). Prussia would never accept it. It was simply too valuable.
What is possible? Well the most obvious one is the Netherlands keeps Belgium. If the Belgian revolt was handled better I believe it is possible for the Netherlands to keep Belgium.
Another one that is reasonable to expect is: the Netherlands is attacked during the first world war and after Germany is defeated is rewarded by gaining small parts of German territory, not unlike Belgium was. Usualy East-Frisia and Cleve is mentioned.
We can even combine the last two, the Netherlands keeps belgium (and Luxemburg) is attacked during an equivalent war and gains some German territory, like Eupen-Malmedy, East-Frisia, Cleves etc. Or you could have an WWI in which France is the agressor and the Netherlands gains French Flanders and Hainaut.
These are possibly the largest gains the Netherlands can make in the 19th/20th century. There are some other possibilities, but they generally end up with a smaller Netherlands (like the Netherlands gains only Flanders at Vienna).
Lets look at the pre napoleonic Europe. Is there any way for the Netherlands to expand? Well yes, kind of. The biggest problem here is that the Dutch republic was barely interested in expanding. So you need a different Netherlands that actualy wants to expand somewhat for some reason. In a way many German counties bordering the Netherlands were Dutch. Cleves, East-Frisia etc were occupied by Dutch troopesand were part of the Dutch sphere of influence. But the Dutch were ok with them being independent, as long as the Dutch could do whatever they want. They prefered Belgium as a buffer state over a Dutch Belgium. If you can change that, you can have the Dutch republic expand both south and east. In my opinion the candidate for this is stadholder Willem II, who wanted to make an alliance with France to devide the Southern Netherlands.
But even in this case you still lack about half of Belgium and I doubt you can gain a lot of German lands this way. I would say just a couple of bordering counties (East-Frisia, Cleves, Bentheim, Lingen, Moers, possibly part of Munster etc.).
In the end I think that a surviving United Kingdom of the Netherlands, which get compensated by a small part of Germany/France after one or two world wars is the biggest you can get.