Oooooh right up my alley (having spent the day writing dog breed related content!)...
Big but short - Grand Basset Griffon Vendeen - scent hound, walk all day, but you'll probably want a long line as recall is a challenge. THey're not wilfully 'fuck you not coming back' on the whole, more 'I am thoroughly unaware you're not right behind me, until I lose this scent...' recall once they REALISE you're not right there is usually pretty good!
The other downside is that boys can be a bit dribbly, it depends on how loose and flappy their faces are.
Upsides - grooming is easy, just keep it knot free, its a wirey/broken coat.
Nature - affable, friendly but not thuggish with strangers, easy going as long as you don't try any force based stuff. The only times I have heard of aggression issues is where people have tried to dominate or chuck their weight around with the dog... and they just won't stand for such handling and it will end badly for all :(
I love mine, he's absolutely the best and gives the BEST cuddles ever... no bother in the showring either, will sell his soul for food, so easy to train in that respect.
Also, every bit as happy to go for a walk as to slope off back to bed for a nap, actually... possibly happier to go for the nap, every single one I know thinks bedtime is the best time of day (next to cuddle time and walkies time and food time).
Another from the hound group - Deerhound..
Obviously a LOT taller, again gentle and easy going unless people behave stupidly and try to use intimidation and aggression with them, though possibly a bit more leeway than the Grand..
Remarkably cheap to feed for such a big dog (not so the insurance which is why i no longer have one), though can be a bit picky...
Grooming needs doing but no skill required just brush it properly.
Training... food works, don't ask for anything particularly clever but I have not had an issue teaching recall, lie down and be quiet, don't jump on people... so the bare basics (if you try teaching clever tricks they tend not to see the poitn and just go to sleep though theres always the odd one that likes it!)
My deerhound was my 'take anywhere, do anything, is always welcome, even by people who think they don't like big dogs' kinda dog... she was ace. And the lifespan on them is not nearly so bad as many giant breeds, mine was 13, many within the breed will see 12 or 13!
Downsides... bloat is a risk, expensive insurance, won't entertain competitive dog sports (but show just fine, mine loved showing, everyone was admiring HER of course!)
Other thoughts..
Most of the really big breeds (barring the lighterweight sighthounds) are too heavy to stay sound long, do long walks etc, or they have a horribly short lifespan and high cancer risk.
I would go for 'tall' rather than 'big' if that makes sense.
Borzois are nice and males are pretty big, theres also lots more tall/hairy/hefty/low/ french scent hounds to look at, Grand Blue de Gascoigne, Grand Griffon Vendeen...
Still in France... Picardy Sheepdogs (Berger Picard) are really nice, if you like the idea of a GSD but not the fear/reactivity issues, and the health problems, the Picardy is described by owners as a mix of Deerhound and GSD... much more relaxed, very very trainable, very affectionate, lovely and funny, easy to live with.. but healthier in body and mind than the GSD.
A nice Rottie is a fantastic dog, the trick is finding one... I have known some absolute belters but there are so many vile breeders, it is not easy.
Of the truly giants, the Leonberger is probably the one I would have, bred for looks and presence rather than desire to DO anything other than woof, impressively... they are typically pretty gentle, happy to walk or not walk...
Downsides, health issues common to giant breeds particularly back and hind leg problems... 3 years to mature, elderly by 9, gone by 11 or younger..
Go to discover dogs, also of the commercial sites with breed guides, the Purina breed library has some decent info (though some of the breed images are from a rather outdated stock image library and seem to have been uploaded by a person who has never met a dog!).
Definitely... get a shortlist then go and meet examples of the breeds IN peoples houses... at shows is nice and better than nothing but to REALLY get a feel for how big a dog is in a house, you need to see it in a house!