As you are Marathi I am too and we are celebrating this Utsav(FESTIVAL) for the last 30 years and before that my grandfather used to celebrate in our village.
As your concern, there is no limitation in size of Ganesh idol, you can go from the smallest 1-2 feet to 108 feet too. There is no Shastra behind the size of Ganesh idol in history. And for proof, there are videos on youtube explaining that size is not important but the placement and direction is!
So you can place ganesh facing east, west.If possible, select north, as it is the abode of Lord Shiva, father of Ganesha. But, when you offer prayers, your face should only be towards east – the vastu preferred direction.
you should never place Ganesha idol or locate pooja rooms in the south or southeast directions. ‘Yama’ and ‘Agni’ rule these directions respectively. So, if you place ganesh idols on these directions, you won’t be getting the blessings you wish for.
Also never place ganesha idol near wahsroom, bedroom, under the stairway.
What we do at home as instructed by our guruji.
bring ganesha idol home covered with red cotton cloth.
Then, Place Red cotton cloth and draw swastika from rice on that, where ganesha idol will be placed.
Then, Place ganesh murti on that place with loudly saying "GANAPATI BAPPA" and then others will say MORYA by this saying joy and happieness is spread all over the house.
The patri is offered to ganesh idol. and kalash is placed on right side of idol.
combination of 21 leaves is called patri.
after idol is placed spread five paan leaves with 1 supari, 1 badam, 1 halkund,1 supari and rice on top.Then put haldi-kumkum on each of them.
And then you can lift cover of ganesh idol.
And then you chant aarti of ganesh "Sukhakarta dukhaharta....."
There are lot sites will tell you the process of puja.
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