One thing to remember about outdoor speakers- small ones sound really crappy unless they're very close to walls, corners or preferably, both. If they're in free space, don't expect to hear any bass- the boundaries are needed to reflect sound toward the area where the speaker is aimed.
First, you need to define the sound quality that you want. If you want loud and full-range, that's possible, but it's not cheap. If you don't have a larger budget, you'll need to manage your expectations, possibly by doing this in stages.
The height is less important than whether some kind of boundary is available- if you have an overhang, use it- it will also provide some protection for the speakers. However, by facing the speakers away from the building, you're effectively trying to provide sound for an area that's far larger than possible, at a decent SPL, so it's better to aim them toward the building- the sound will reflect off of the walls to reinforce the sound level and it prevents sound going toward the neighbors or street (which isn't the purpose of installing them).
You don't need stereo and in fact, you don't want it unless you're setting up an outdoor listening area specifically for stereo. For covering a large area, which is called 'distributed audio', mono is best because there's no ping-pong of sounds that are only in one channel but coming from speakers that are very far apart (listen to old Beatles songs with headphones for an example) and if you're directly across from one speaker, it's highly likely that you won't even hear the next speaker, which may be providing almost half of the sounds on the recording.
Amplifier power- whatever your receiver can produce, it won't be enough. If overall output is less important, think about some kind of outdoor subwoofer because it's the bass where small outdoor speakers fall on their face.
This is one example of a speaker system
type that can work, but these are not all equal- some aren't very good, some are great- the great ones are expensive, but you won't really think you're listening to outdoor speakers.
If you want to do this and not spray sound all over the neighborhood, think about using something that's made for outdoor sound, which direct the sound to a narrower area. Small PA speakers will do this and many brands are out there- JBL Control System (C-28-i), Atlas, Cerwin-Vega, MTX and others. Most are available in white or black and the ones in plastic enclosures are paintable. The JBL come with a mount.
The best outdoor systems come with a power amp that has a DSP and the equalization is set for outdoor use- it uses one channel for the sub(s) and the other channel uses a 70V system, which is normally part of the commercial/industrial realm, but it allows mono operation and connecting up to 8 speakers to one channel's output (it's designed to connect the speakers in parallel, as long as the speakers have the appropriate transformer setting) and the Sonance, Niles and some of the others sound great- the sub is buried "up to its neck" and the satellite speakers look a bit like Malibu landscaping lights.
THis one doesn't come with an amp-
https://www.proavdealer.com//sonance-sr1a-outdoor-speaker-system-with-sub.html?gclid=EAIaIQobChMI6-vphdDk2gIVkolpCh2f5Q8fEAQYBCABEgLO8fD_BwE
This system has an amp-
http://www.sonance.com/outdoor/sonarray
Your climate will determine the longevity of the speakers.
