They say pressure is mounting for new recommended limits after years of coping with rising numbers of pupils. The Secondary Heads Association is to canvass the views of its 8,000 members .
It could mean members being recommended not to arrange lessons of more than 30 pupils for classroom subjects or from 20 to 25 in practical classes such as science and design. Until now the union has said staffing and class size matters are up to individual members.
The National Association of Head Teachers says schools should be given enough money to reduce class sizes, but does not specify any limits.
New figures from the Chartered Institute of Public Finance and Accountancy (CIPFA) show that the number of teachers has fallen sharply while pupil rolls have increased.
Peter Miller, vice president of the SHA, said: "We are moving towards the idea that there should be recommended maximum class sizes. We as managers have tended over the years to squeeze in a few more children here and there and make it work.
"But the time is coming where we may have to take a stand. We have our staff's mental and physical health to think about."
CIPFA's figures, based on budget assumptions made by LEAs, show there is on average one more pupil for every teacher now compared with three years ago.
Phillip Ramsdale, joint managing director of the organisation's commercial arm, said: "There is a clear trend in the reduction of overall numbers of teaching staff and an increase in the number of pupils. The resources just haven't kept pace with rising school rolls."
The number of teachers in LEA schools in England and Wales has fallen by more than 10,000 since 1993 and now stands at 369,200, according to CIPFA's estimates, while the number of pupils has risen by 14,000 over the same period to 7,156,000.
The report, which was published this week, is based on responses to a CIPFA survey from 103 of the 117 education authorities in England and Wales. It shows there are 16.4 pupils for every teacher in secondary schools now compared with 16.2 last year.
Budgets set by LEAs at the beginning of this financial year represented a 3.6 per cent rise over the previous year's spending, but when what authorities actually spent is taken into account the increase is only 0.03 per cent. And if pay and inflation are considered, CIPFA says, this adds up to a cut in real terms.
The report comes after figures last October showed the number of primary pupils in classes of more than 30 had risen to more than a million.
Meanwhile Conservative and Liberal Democrat education spokesmen at the Association of Metropolitan Authorities have joined with Labour education committee chair Graham Lane in warning governors of bleak prospects for school finances.
They have told governors that while the Government has announced that an extra 4.5 per cent will be spent on schools next year, the funding formula meant an increase of only 2.1 per cent in the standard spending assessment - what the Government thinks local authorities should spend on education.
This means the 4.5 per cent increase can be achieved only by making massive cuts in other council services, they said.
The Liberal Democrats' education spokesman, Don Foster, said CIPFA's figures disproved the Government's claim that it is providing extra money to fund education.