Brookfield in 1917 was a hundred acre farm tenancy, as was Grange Farm. Brookfield had Lakey Hill, suitable only for sheep, as part of it; Grange Farm was on the rich land by the river, and considered better and likely to be more profitable.
Source: The Book of The Archers written by three members of the cast with the blessing of the editorial team in 1994. Information about the various farms is also in Jock Gallagher's 1988 trilogy of novelisations: To the Victor the Spoils, Return to Ambridge and Borchester Echoes.
The TBoTA entry on Brookfield starts with the words "The Archer family had been farming Brookfield for many years as tenants of the Lawson-Hopes when in 1917 Dan Archer, at the age of twenty-one, succeeded his father in the tenancy. The farm was then 100 acres."
When Dan bought his farm during the Lawson-Hope estate sell-up in 1954 it was still 100 acres. He was able to do this because he had always stayed in credit at the bank and had saved money "against a rainy day"as a matter of habit, so he was able to take out a mortgage.
Dan then went into a partnership with another farmer, Fred Barratt, in 1962, and founded "Ambridge Farmers", which lasted until Fred wanted to give up farming and retire. Meanwhile Phil (who had been working as Estate Manager for George Fairbrother, who owned a farm in Ambridge between 1951 and 1959) had bought Allard's Farm, which was also in Ambridge Farmers, and in due time that became part of Brookfield, when Phil, Jill and their four children moved there in 1969.
During the sixties Dan and Phil bought parcels of land whenever they were able to, and gradually increased the size of Brookfield. There was a setback when Dan died and Phil had to sell 55 acres of land to pay death-duties, but between them they increased its size to 469 acres.
Joe Grundy's father George, meanwhile, was a bad farmer who had no money saved against a rainy day and no credit with the bank, so that Joe was unable to buy Grange Farm when the Estate was split up. Over the succeeding years Joe let Grange Farm fall into such a bad state that it became impossible to farm it at a profit, and eventually went bankrupt and was evicted about thirty years after the first notice to leave for non-payment of rent had been delivered to him in 1970.