In a letter posted on Facebook, Ruth Clemens complained that her experience of trying on a pair of the store's jeans in a size 16 - the largest size it stocks - "did not go well".
She posted a photo showing the jeans were significantly too small for her body, which she explained usually suited size 14 trousers and medium tops.
She wrote: "I'm normally a size 14 on my hips (occasionally 16 if buying trousers) so I thought I'd try them on. It did not go well."
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"I am not overweight (not that that should matter) and although I'm 5 foot 11 my body is pretty average shape-wise. It's already difficult enough for me to find clothes that fit well because of my height, why are you making jeans that are unrealistically small?"
"Am I too fat for your everyday range? Should I just accept that accessible and affordable high street and on-trend fashion isn't for people like me?"
She added: "You might recognise the top I'm wearing - it's one of yours and it's a size medium. Sort it out would you."
Ms Clemens' post, which has attracted well over 6,000 comments, follows numerous debates on social websites such as Mumsnet about H&M's sizing.
H&M has stores in Sunderland, Durham, the MetroCentre, Newcastle, North Tyneside and Hartlepool.
In its online guide, H&M advises that a medium equates to a UK size 14-16 for a waist measuring from 29 inches, while large equates to a UK 18-20 or a waist measuring from 32 inches.
Marks and Spencer advises that its size 14 is for waists measuring 31 and three quarters inches, a size 16 is for 34 inch waists and a size 18 fits a 36 and three quarters waist.
H&M replied to Ms Clemens' post, explaining that sizing can vary depending on the style, cut and fabric.
The retailer wrote: "We are sorry to hear about your experience in store recently. We always want our customers to have an enjoyable time when shopping in store and to leave feeling confident in themselves."
"At H&M we make clothing for all our stores around the world, so the sizing can vary depending on the style, cut and fabric. We value all feedback and will take on board the points you and other customers have raised."
An H&M spokeswoman said: " Everyone is welcome at H&M and we value all feedback."
"H&M works with the same measurements and sizes within all concepts, across all 62 markets and online; however, the fit of a garment may vary depending on the style, cut and fabric."