Brief Answer:
Metfomin dose

Detailed Answer:
Yes it is indeed true that metformin can be taken in doses as high as 2500 or even a bit higher than that, provided there are no reasons to prescribe lower doses such as:

Intolerance (for example, stomach related side effects like nausea, vomiting, diarrhoea)

Lower than normal kidney function (which would reflect in a blood test called serum creatinine)

Heart or Lung or Liver failure

Low blood pressure/oxygen states like blood loss from accidents/ ICU patients with widespread infection

So barring these, optimal doses of metformin can be close to 2000 to 2500 mg per day in two to three divided doses, generally taken around meal times.

Now an equally important question to ask in your case is whether or not you are a candidate for this medication from a medical perspective.

Surely metformin has been shown to be effective in preventing progression of pre diabetes to diabetes. Insulin resistance is the main cause of pre diabetes and plays an important role in diabetes too. Likewise, it is very well associated with PCOS too.

So many medical practitioners offer it to patients with these conditions.

However newer guidelines have laid out systematic criteria in PCOS regarding metformin use. Prior to this, it was fairly common to prescribe metformin for those with PCOS.
These Endocrine Society guidelines which are respected worldwide recommend using metformin in PCOS only when there is evidence of pre diabetes i.e. HbA1c between 5.7 to 6.4%
Your A1c is 4.2% so technically it is normal.

Of course, if diabetes is present in a person with PCOS, metformin is the preferred medication as long as there are reasons not to prescribe as enumerated above.

But these are only guidelines and individual doctors are free to choose medications options from personal experience

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Brief Answer:
Follow up

Detailed Answer:
The exact basis for PCOS continues to baffle scientists worldwide even today. I am presenting an oversimplified mechanism for the underlying scientific cause of PCOS. Excess male hormones ('hyperandrogenism') are made by ovaries in most females with PCOS and causes the cardinal clinical features of the syndrome. About half of patients have insulin-resistant hyperinsulinism (high blood insulin levels), which results in the ovaries making more male hormones and contributes to obesity. Male hormone excess may also cause LH (Leutinizing hormone form the pituitary) excess, which cause the ovaries to make even more male hormones in the presence of hyperinsulinism. Obesity increases insulin resistance, and the resultant increased insulin levels further aggravates hyperandrogenism.

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Brief Answer:
Second follow up

Detailed Answer:
Metformin has been found to helpful when the insulin resistance has manifested as either pre diabetes or diabetes

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