It claims that the Test Valley Borough Council-commissioned report on the future shopping needs of residents suggests that a future town store would not need to be of the size proposed by the supermarket giant to cater for Romsey.
Tesco wants to build a 35,000 sq ft - or 3,252 sq m - store on a six-acre site known as Fairground Field, on the Broadlands Estate.
It says it will provide 200 jobs and draw people in to the town benefiting town centre businesses.
However, opponents fear that the store will cause congestion and harm trade in existing shops.
The Nathaniel Lichfield and Partners report will help inform plans for future retail developments in the borough and forecasts the amount of 'retail floorspace' that could be supported in Romsey and the town's future needs.
This is calculated on existing demand from the Romsey population and additional demand due to population growth.
It states that, from 2016 to 2021, Romsey would require a food store size of between 1,100 sq m and 1,300 sq m for main and bulk food shopping.
David Willox, joint president of Romsey Chamber of Commerce, said: “The report makes it clear that a store of the size proposed would have a significant impact on the town centre and it's also clear that a store of this size is not required.”
Mr Willow said he believes that overall to attract a supermarket chain in, to satisfy that supermarket's inevitable demand for non-food items and to stop people continuing to leave Romsey to shop a total floor space of 2,000 sq m up to 2,500 sq m would actually be justifiable.
He said his discussions with other supermarkets had arrived at a similar figure and that if too small the store would end up competing with Romsey's other supermarkets.
He added that another important aspect of the report was its appraisal of the National Planning Policy Framework which now requires councils to pursue policies that support the vitality and viability of town centres.
However, he said he was concerned that alternative sites to Broadlands had been dismissed mainly on the grounds of cost.
Tesco refused to comment on the size issue but pointed out that the report found there were not any suitable sites for a supermarket close to the town centre of the size it is proposing.
Tesco corporate affairs manager, Simon Petar, said: “We are pleased that the findings of this independent study agree with our own assessment.
“Namely, that there is no suitable, available and viable site in the town centre which could accommodate a store and car parking of the size needed to attract customers back to Romsey.”
A Test Valley spokeswoman said the report would help inform the council's decision making as “additional evidence which is taken into account when considering and planning local developments”.
She said the brief given for the study was a broad assessment of the viability of potential development sites.