Check online, they have a price & size limit guide (parcel to be up to a certain size but no more than 30kg).
Best if you have a tape measure and bathroom scales or similar to get the weight.
I think their limit is 1.5m long / 3m for length & girth combined.
Be accurate when booking or getting a label as anything over the above measurement has to go on their 48 LARGE service. I've heard of some people getting up to £90 surcharged as they under-declared the weight / size (not sure about contract customers as they have some privileges).
As said, get a tape measure, bathroom scales (or other to weigh the item), and check on their site under quick quote (if too big / heavy, this will tell).
And always make sure the item is very securely packaged, with the address details inside the box as well (as informed by a member of their staff, what if a tin of paint fell onto box - no address details, but if they open it, they can resolve delivery in most cases). All info is on their website.
Hope this helps.