Hi Debra,
There is no limit to the size of any tree, however there is a 'duty of care' that a tree owner (the person whos boundry the tree is within) has to property or persons that the tree may harm or damage.
Also any branches over hanging your boundry (ususally the wall, but better to check deeds for absolute certainty) can be cut by yourself or your appointed contractor, as long as they are returned to the neighbour in question.
So if you think the trees are going to cause damage to your property or persons you could speak to your neighbours and inform them that they have a duty of care and that it may be better for them to pay for works to the tree rather than the expense they will be liable for should the tree damge your home (as they would have to pay the full costs of this, and then rectify the tree) or tell them that you are going to have the tree cut on your side of the boundry and you will return the debris to them. Usually a high majority of the cost of tree works is in the removal of waste, and as you are legally obliged to return the debris to them so it may be worth coming to an arrangement between you in the first place, as it may cost them to have the debris removed. Potentially you could let the contractor remove the debris for you , as long as you have agreed it with your neighbour first.
It potentially would be worth having an estimate (most firms offer these free) to get an idea of costs and present these to your nieghbours and offer to split the fees to make it work for everyone. If this doesnt work, the arborsit will have let you know if they think that there is a potential for damage from the tree, and perhaps letting them read a duty of care in trees document from the internet will let them understand their potential liability. This really should only be brought up if they refuse to work with you as it is quite a confrontational stance.
For Further Guidance please follow this link to the Royal Forestry Society
http://www.rfs.org.uk/learning/common-law
http://www.barrelltreecare.co.uk/pdfs/BTC42-RICS-Complete-DutyofCare-210510-HR.pdf
Matt Davies
www.forsythgardening.com
0131 466 7134
