The spill - the worst in the region for more than a decade - began at the Gannet Alpha platform 112 miles east of Aberdeen on Wednesday.
Estimates have only now been released by the firm, prompting complaints from environmental groups that there has been a lack of transparency.
Shell said about 216 tonnes of oil, equal to 1,300 barrels, may have spilled into the sea so far.
The estimate outstrips annual spill totals for the past decade, according to figures from the UK Government Department of Energy and Climate Change (DECC).
The total amount of oil discharged into the North Sea in 2009 was 50.93 tonnes.
In a statement today, Glen Cayley, technical director of Shell's exploration and production activities in Europe, said: "This is a significant spill in the context of annual amounts of oil spilled in the North Sea.
"We care about the environment and we regret that the spill happened. We have taken it very seriously and responded promptly to it."
Mr Cayley, speaking from Aberdeen, added: "The high winds and waves over the weekend have led to a substantial reduction in the size of the oil sheen as can be seen from the current levels on the water.
"We continue to expect that the oil sheen will disperse due to wave action and that it will not reach the shore."
Shell said the current rate of leakage is under five barrels a day. About one tonne is estimated to be on the surface, although that figure changes from day to day.
The surface sheen, which also changes daily, is about one-third of a mile in size. The spill is described as a light crude oil with a low wax content. Hydraulic fluid is also present.
A standby vessel, Grampian Prince, is monitoring the area, with oil spill response equipment available if required.
A spokesman for the DECC had earlier estimated the spill to be "several hundred tonnes".
The spokesman said: "Although small in comparison to the Macondo, Gulf of Mexico, incident, in the context of the UK Continental Shelf (UKCS), the spill is substantial - but it is not anticipated that oil will reach the shore and indeed it is expected that it will be dispersed naturally."
The spokesman added: "Current estimates are that the spill could be several hundred tonnes.
"However, it is always very difficult and takes time to get an accurate assessment of the size of a spill and this is subject to ongoing revision."
The Scottish Government said it will push for a full and formal role in any investigation, given its responsibility for the marine environment.
Scottish Environment Secretary Richard Lochhead said: "While there are inevitable difficulties verifying the extent and size of the leak, it's vital that Shell and DECC make information available on an open, transparent and regular basis."
Environment groups raised their concerns for wildlife in the area.
Stuart Housden, director of RSPB Scotland, said: "We know oil of any amount, if in the wrong place, at the wrong time, can have a devastating impact on marine life. Currently thousands of young auks - razorbills, puffins and guillemots - are flightless and dispersing widely in the North Sea during late summer. So they could be at serious risk if contaminated by this spill.
"If Shell is confident that the situation is now under control, we must now start to assess what happened and make sure the relevant precautions are in place to stop this happening again."
Greenpeace oil campaigner Ben Ayliffe said: "There is a worrying lack of transparency from Shell in relation to this oil spill and our concerns are only heightened by the news that this spill is now substantial.
"It took Shell two days after the spill began before they admitted that there had been a leak, and even now they have refused to say how many barrels of oil per day are seeping into the North Sea.
"Given this lack of openness, you have to ask if Shell is the right type of company to be allowed to expand its oil operations into the environmentally-fragile Arctic."
Juliet Swann, head of campaigns at Friends of the Earth Scotland, said: "We are deeply worried that we still, even five days after the leak was detected, know far too little about the environmental impact of the spill, how it could impact wildlife, and the scale of the threat to Scotland's coastal communities and the marine environment that they rely on for their income."
She said Shell's response "has been close to pathetic" and added: "It is Shell's responsibility to keep the public and stakeholders informed, especially in a crisis such as this, but I fear the longer we wait to hear about what is really happening from Shell, the more chance there is that we will never actually know the truth as Shell's PR machine goes into overdrive."
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