OK sounds good.
Here is my suggestion the Doge's Palace - it's a very famous and big building which is next to the St Mark’s Basilica.
I attached two pictures of it one that I took and one from google earth. To see more details of the building you can google it or go to google earth there you can see it as 3d building.
The rules for modelling are: not more than 2000 polygons for the whole building, so don't make it too detailed.
And if you want to texture it you have to use a 512x512 pixel texture map, if you don't want to do it, I will texture it.
I think it would be the best to show some screenshots of your model before you upload it.
Please let me know if it is okay so, to avoid complications.
Joey