From a Hungarian perspective there probably is no 'fair' division of the country; any division would have taken away historic Hungarian lands, parts of the Hungarian Kingdom for over a millenia, an idea to which virtually all Hungarians would have been opposed. The Kingdom of Hungary had a much stronger inner cohesion and historical consciousness than the Austrian Empire taken as a whole: Galicia and Croatia, for example, had pretty much nothing to do with each other, apart from both being annexed at one point to the ever expanding Habsburg realms; Hungary has however existed as a unified Kingdom for centuries.
Yet divison was inevitable. Ethnic minorities made up about half of the total population (without Croatia) and they mostly lived in compact groups where they made up majorities: Slovaks in the North, Romanians in Transylvania and Serbs in the South- all these people wanted either a country of their own or to join their already existing nation-states. Preserving such an anacronistically diverse Hungary in a Europe obsessed with ethnic identity would have probably been impossible either way.
What would have been fair, though, is to draw the political boundary according to the ethnic one, without such considerations as strategic railways or rivers with military importance- which was what happened in reality. The most striking unjustice was done with the Czechoslovakian border (Slovakia and Carpatho-Ruthenia of Ukraine now). Here, for reasons such as the strategic importance of the river Danube the border was drawn significantly souther than the Slovak-Hungarian ethnic boundary, through compactly Hungarian-inhabited lands. About 90% of the ~1 millon large Hungarian minority in Czechoslovakia became thus concetrated in a narrow strip along the border where Hungarians made up an 80%+ majority. Had the border respected the ethnic boundary, there would be virtually no Hungarian minority in Slovakia and Ukraine today. Whether the cities along the border, most notably Bratislava and Kosice, but also Nitra, Mukachevo and Uzhhorod could have gone to Hungary is debatable, but at the time they were all Hungarian majority (save for Bratislava which was ~40% Hungarian and ~40% German , but would almost surely have voted for Hungary).
In the south, some territory along Subotica, as the town itself, should have probably gone to Hungary as well; but in the most interesting part, in Transylvania, the situation is also the most complicated. Here, the largest Hungarian-majority area, the Szeklerland, where almost half of the Hungarians in Romania live, is in the easternmost part of Transylvania, separated by Hungary by large Romanian-majority lands. I just see no way in which this land could have been awarded to Hungary in any scenario; at least as many Romanians would have to go to Hungary as Hungarians, so from the point of view of ethnic justice nothing would be accomplished; instead Transylvania would just be divided in a very unnatural way, as it briefly happened indeed after the heavy-handed Vienna Dictates of Hitler.
Adjustments could have been made along the border, however. Here, again, strategic infrastructure and the importance of the chain of border-cities- Satu-Mare, Oradea, Arad- pushed the border more to the east than the ethnic makeup would suggest. The Satu-Mare area would have definitely gone to Hungary in a 'fair' deal, and in a particularily pro-Hungarian version maybe even Oradea and -pushing it- the Salaj region, where the ethnic balance was more equal.
Adding up all these adjustments there would have been about +1 million Hungarians left in Romania and less than 1 million in all the other lands added together; a significant number still, but at least 1,5 million lower than in reality, at the cost of keeping a minimal number of Slovaks, Romanians or Serbs within Hungary.
