Hungary's largest trees are catalogued in this webpage. The registry is based on my book "Hungary's Largest Trees - Dendromania" (Alexandra, 2005). The tree lists are continuously expanded/actualized, and anyone can contribute - in a controlled way.
What is big?
The original goal was to have a full list of trees having a >6 m girth. Not counting the hard-to-catalogue poplars and willows (they grow fast, die fast, and many of them are found in the impenetrable, jungle-like flood areas along the large rivers), there are about 200 such trees in the country, mostly oaks, limes, sweet chestnuts, planes. However, the lists have been expanded to include smaller trees as well (e.g., oaks, limes, chestnuts down to 500 cm, beech to 400 cm). Genera not reaching impressive sizes are not systematically surveyed, but stumbling on a specimen considered to be extreme large in its genus, it is put on the list.
There is a mention of the site in American Forests Winter 2011 issue:
Hungarian Global ReLeaf
Revisiting the first Global ReLeaf International project.
—by Gyorgy Pal Gado http://www.americanforests.org/productsandpubs/magazine/archives/2011_Winter/20%20Years%20of%20International%20ReLeaf.pdf