You may want to play with landscaping software to estimate area requirement http://www.toptenreviews.com/software/home/best-landscaping-software/

Wow they're all so freakin expensive... any free softwares??

I wouldn't really worry about landscape design software. Just go with your gut and plant where you want as long as it will work long term. I have no landscape "plan" and it isn't necessarily organized. I have lots of plants, my goal is to have as many different plants in the space as possible but it still look beautiful. Here are some more plants you might want to check out:

acer palmatum 'crimson queen'

acer palmatum 'waterfall'

cercis chinensis 'avondale'

cercis canadensis 'royal white', 'rising sun', and 'merlot'

craetaegus virdis 'winter king'

abelia x gradinflora 'rose creek'

lagerstromeia indica x faurei 'natchez'

rhododendron 'roseum elegans'

hibiscus moschetous (probably spelled wrong, perennial "dinner plate" hibiscus)

variegated english holly

cornus kousa 'wolf eyes'

cornus kousa 'satomi'

magnolia x soulagnea

Those are not all japanese, but they fit well with the look and some are at least closely related.

Wow the cercis chinensis caught my eye! I didn't know there was a redbud species existed in Japan.. I will consider the plants you listed but for Japanese maples, I tend to stay away from dissectum species. I planted a waterfall last summer in my backyard but probably won't for my future home. I like the natural looks of upright Japanese maples. And the reason why I selected fireglow, hogyoku, and aoyagi is because fall colors for those trees will always be red, orange, and yellow, respectively. I also don't expect to buy a huge property so I'm trying to keep my list of plants limited so it won't run out of space

If the OP is still interested in his own question of how big his yard should be, then the only way to estimate that with any degree of accuracy is by way of landscape design and try to minimize the area.

I'm not sure I fully like this strategy. Gardening is VERY location dependent. If I end up buying a seasonal swamp I'll plant tupelo and bald cypress...if I end up on a sandy slope I'd plant different trees. I also don't like removing all pre-existing plant life. Many trees take more time then most people live on a property to achieve true majesty. If everyone does that you end up with only dinky trees. And some of the trees you remove may be someone else's lifelong dream. You should be able to find a lot that was cleared out by lazy developers, though.

Pinus thunbergii is plagued by disease and doesn't live long in my area.

kudos for thinking so far in to the future at your age! My kids think money comes from 'mom's wallet'. Just keep in mind that your focus may change over time, especially once you start planting and figuring out what works and what doesn't. Draw a picture now of what you are thinking so your future self will recall.

"I'm not sure I fully like this strategy."

LOL!! Ed makes a very good point. Typically this situation is addressed the other way around - you find the property/house you like, with the right location and can afford and then figure out how to landscape it and with what!! As noted, many of these plants are rather location specific and site specific in terms of soil conditions and sun/shade conditions.

Not saying this can't work - only that it may not produce results that you are happy with. Good landscape design involves a great deal more than just plant selection!! And design software will get you nowhere fast unless you have a property in mind to begin with.......that is the foundation/basic platform for any landscape design software.

my only comment is to do research on each different tree you want.

My advice is to be flexible,. And think about living in rural areas. Near the big city a lot big enough for all these plants would be within the reach of only the very rich. Location matters.

The OP could do his dream landscape design by hand with pencil and paper, without software assistance nor the built-in knowledgebase. He could do many, many design options for clay or sandy soil, lakeside or mountaintop, suburban or rural, etc.

He wants to be a landscape designer anyway.

Yes I am actually going to be very very picky when time comes for us to choose a house with the real estate agent. I also already know which town I'm going to live in. Its only like 10 minutes away from my current house. Here's my plan and restrictions:

Small house. Big enough front+back yard.

House must be in full sun(no shaded by big deciduous trees)

Land is flat. No slope, no hills, no crater, no downhill. FLAT. Which means the soil will most likely be well draining. Unless its clay

Have very little pre-plant life. All grass yard is fine for me

No big deciduous trees on property(black walnut cannot be even near my property)

Preferably no pre-existing conifers.(cedars are ok)

Have space to make bamboo hedge on right and left side of property and backyard

No vole population

So yea I'm already semi-planned for location wise. Only problem is IF I can find such house...

Well, then, there is your dream site to do your dream landscape exercises!

Good luck!