Their base height is from their issued Pokédex information, however because of the formulas, they'll never be exactly their normal size (especially only Lv1). Let's get started.

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Base Height is numerical in inches (ex. Pikachu is 1'04" or 16 inches, use 16 for the base height.)

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HP
*yourHP multiplied by 0.165
*Number received is a Percentage (Rounded to the nearest tenth) that you'll add onto the Pokemon's height.
(ex. 16 + 8.2% = 17.312 or 16 x 1.082 = 17.312; also keep height rounded to nearest tenth)
*You're gonna need to hold onto the new height throughout this formula

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[i]ATK
*yourATK multiplied by 0.07
*Round the number to the nearest tenth to take that exact number as a percentage to once again add to the height.
*Add the percentage to the new height from your HP stat
*I don't expect the numbers to be even, so always round to the nearest tenth (Until I say).
*Store the new height and prepare for the next step...

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SpATK
*yourSpATK multiplied by 0.07
*Round to nearest tenth to make it a percentage to add to height from ATK
*Keep everything from before in mind and hold onto the new height...

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DEF
*yourDEF multiplied by 0.07
*Round to nearest tenth to have your percentage to add to the height even more.
(Refer back to HP on how to sum the height)

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SpDEF
*yourSpDEF multiplied by 0.07
*Round to nearest tenth to have your percentage to add to the height for the last time...
*Now with your height still in inches from the beginning, remove the tenth to a whole number by rounding either up or down.

This whole number is your Pokemon's height based off its stats. But we're not done...

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SPD
Yep, we're doing its Speed stat too. But it's not going to affect the height, it's going to change its weight.
"But how do I find the new weight based on the height it has?"
*Start off by dividing the new height by the Pokédex height.
*Don't round this number. Instead, cube it (x³), hopefully you have a calculator somewhere.
*Now take that cubed number and multiply it by the Pokédex weight of that Pokemon. (It's a lot heavier now isn't it?)
Well don't worry rhetorical question! That's where this Speed stat will work for you & your Pokemon!
*yourSPD multiplied by 0.185
*This number you have now is yet again a percentage, round it to nearest tenth. But this time you're subtracting it, not adding.
*Subtract the percentage from the new weight you found earlier and round that to the nearest tenth.

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You now have both height and weight based on stats. Say hello to your embiggened partner!