Could it be a Bur oak?

Not likely to be your candidate, but Quercus dentata (Daimyo Oak, from Japan) can have leaves 40cm long and 20cm broad. Should be hardy zone 6 certainly, and probably 5.

Resin

2.54 cm = 1 inch

John,
Shade leaves on seedlings or stump sprouts of many oak species can be huge. Of the natives, however, bur oak typically has the largest leaves - a foot in length is common. And, the acorns, particularly from specimens originating in the southern/western portions of its range, can be huge. I first noticed bur oaks when I lived in mid-MO, and was attracted initially by the large acorns, with their ornate, frilly caps. But, in the intervening years, I've accumulated seedlings from around the country, and have had some acorns collected or sent by friends, from AL, TX, OK that will run in the neighborhood of 6-8 per pound - with caps removed!

Very large leaves in that zone certainly suggest Quercus macrocarpa. A white oak group member though. Were the leaf lobes pointy or rounded?

+oM

Pointed leaves- it was some kind of red oak. I asked about both reds and whites mainly out of curiousity. Do any red oaks have leaves longer than northern red oak - ones that can grow in Iowa?

john

Northern Red has the largest leaves of any of the hardier red oaks that I know of.

Resin

Blackjack oak (Quercus marilandica) is in the red oak group and can have some leaves that large, particularly on coppice and understory growth. However, Iowa is outside the species' natural range, and its acorns are rather small.

Actually Blackjack Oak is native to Southern Iowa.

The only options for Red Oaks native to Iowa are Shingle Oak(Q. imbricaria), Northern Red Oak(Q. rubra), Black Oak (Q. velutina), Blackjack Oak(Q. marilandica), Pin Oak (Q. palustris), and Hill's(Northern Pin) Oak(Q. ellipsoidalis). I'd eliminate Shingle Oak based on the leaf shape described, Blackjack Oak is rather limited in range in Iowa, and unless your relatives live in Southern Iowa(mostly the counties bordering Missouri), I'd eliminate it also. So, that leaves you with only 4 choices. Out of those, Black, Northern Red, Pin, and Hill's, Black Oak on average(there are always exceptions to the average) has the largest leaves. But I'd say Black Oak is your most likely candidate because of it's common occurrence in Iowa as well as it's larger leaves than the others on average. On vigorous saplings and shade branches I've seen Black Oak leaves that were commonly 10"-14" long.

I will say that the "large acorns" comment by your grandparents suggests Northern Red Oak more than Black Oak though. If this Oak is growing in a dry upland condition then it's most likely Black Oak, but if not it's probably Northern Red Oak.

If the parent tree was planted, it's possible it could be Shumard Oak. They are reported to be hardy in IL, OH, and MI. Shumard oak also has some of the largest acorns of the red oaks I have seen.

The largest "Red" oak acorns that I come across are N. Red oak. Some are nearly Bur oak size. Shumard has somewhat smaller (but still large) acorns.

Shade leaves on N. Red oak can be huge.

I saw several Quercus with leaves at least 1' in lenght. They were cultivated trees in Germany. Very large trees!!. What can they be?

osprey

it is a bur oak