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As noted here:
http://www.selenic.com/pipermail/mercurial/2010-February/030068.html
This is probably documented somewhere, and there may even be a good reason for the difference, but old-style classes raise TypeError when __len__ returns a non-int, whereas new-style classes raise OverflowError. The latter is probably just as valid, but the message is a bit obscure for debugging purposes.
Maybe this went away after 2.5 - if so, sorry for the noise!
Here's an illustration of the problem:
Python 2.5.4 (r254:67916, Nov 4 2009, 17:59:46)
[GCC 4.1.2 20080704 (Red Hat 4.1.2-46)] on linux2
Type "help", "copyright", "credits" or "license" for more information.
>>> class C:
... def __len__(self):
... return 2**35
...
>>> c = C()
>>> len(c)
Traceback (most recent call last):
 File "<stdin>", line 1, in <module>
TypeError: __len__() should return an int
>>> class C(object):
... def __len__(self):
... return 2**35
...
>>> c = C()
>>> len(c)
Traceback (most recent call last):
 File "<stdin>", line 1, in <module>
OverflowError: long int too large to convert to int

The following is tangentially related:
http://bugs.python.org/issue2690 

I fail to see the bug in this report. What did you expect to happen instead?

I would have expected a more accurate error message for the new-style class. In the original message which brought this to my attention, the cause was not particularly obvious:
http://www.selenic.com/pipermail/mercurial/2010-February/030066.html
I concede that the different mechanisms in place for new-style classes might make it hard to have a specific error message in such a situation, though.

In the example you give it seems to me that the message for the new style class is more accurate and useful than the message for the old style class. Looking at your mercurial example, it looks like the problem is the same (len returning a long), and again it seems to me that the new message is more accurate than the old, and that neither one is more confusing than the other in the given context(s). Or, if anything, that the OverflowError is less confusing as it has more connection with the problem (committing *large* amounts of data).
I don't see this as a bug in any sense, myself. What would consider to be a "specific" error message? Perhaps this is a request for an enhancement, instead.

I don't disagree that OverflowError describes what's happening, but the need to convert to an int in the first place is a detail of the machine - you'd have to know that this is a limitation of whatever internal "protocol" CPython implements - not a description of the cause of the error, which is what the old-style message describes quite clearly.
On the subject of whether __len__ should be able to return long integers, GvR seems to like the idea (from the related bug mentioned earlier):
http://bugs.python.org/issue2690#msg70525
I'll take a closer look at the mechanisms for error reporting around this situation later, but my assertion is that the new-style message isn't as helpful as the old-style one.

Interestingly, this (len returning something larger than ssize_t) has not been fixed in python3.
On the other hand, I still think the new-style message is better. Yes, it is an implementation detail of CPython, but that is exactly the error being reported. Another implementation might successfully return the value.

Actually, in the issue reported, the initial problem occurs in the evaluation of an object in a boolean context (and the subsequent problem occurs with an explicit len invocation):
http://www.selenic.com/pipermail/mercurial/2010-February/030066.html
Presumably (from memory and a brief look at the reference), when "if data:" is evaluated, Python attempts to invoke an instance's __nonzero__ method or its __len__ method. Since the mercurial.httprepo.httpsendfile class only provides a __len__ method, the __len__ method's return value is used to determine truth.
The following demonstrates this particular issue:
>>> class C:
... def __len__(self):
... return 2**35
...
>>> c = C()
>>> if c: pass
...
Traceback (most recent call last):
 File "<stdin>", line 1, in <module>
TypeError: __nonzero__ should return an int
>>> class C(object):
... def __len__(self):
... return 2**35
...
>>> c = C()
>>> if c: pass
...
Traceback (most recent call last):
 File "<stdin>", line 1, in <module>
OverflowError: long int too large to convert to int
Here, I could actually argue that the message mentioning __nonzero__ is obscure: there isn't such a method defined, and __len__ is the misbehaving method. Still, in the context of boolean evaluation, the OverflowError is less helpful than it could be.

The exceptions cannot change in 2.7.
For 3.1.2
class C:
 def __len__(self):
 return 2**35
c = C()
len(c)
# gives
OverflowError: cannot fit 'int' into an index-sized integer
Maybe (#2690) that will change.
So I do not see any valid bug or feature request issue.