So, you want to take one of the world's largest empires. An empire that ruled a fourth of the world and add more to it!? !
Why not?
That said, have the US less interested in keeping Chinese markets open and at least part of China becomes a UK colony. Or, the British capture Afghanistan when they first try to. And of course there's the obligatory "US loses the ARW" scenario, but I have my doubts that Britain would get as big of an empire if it was that focused on NA.
The last point is probably very accurate and something that a lot of people tend to overlook. A better route for size is probably a more successful 1812 fight-back, leading to a series of clashes in N America keeping the US trimmed back and possibly split up into a number of successor states. Canada expands to cover most of the west of the country but the threat from the US means its more in favour of imperial federation. Also the frequent clashes, along with concerns in Europe mean that Britain doesn't coast as it did during the 19thC and continues to be an industrial and technological leader longer.
At the same time the initial loss of the colonies does mean attention is spread wider. Britain hence makes the gains in Africa and Asia it does OTL but in the longer run being more powerful it ends up with more.
You would have to either drastically change the Victorian viewpoint or have a total collapse of China to get big British colonies there. The reason Britain put so much effort into keeping China united and reasonably independent was because they realised it was better to trade than rule as the latter is a lot more expensive.
Similarly with Afghanistan. There are plenty of places I would much rather Britain have gained as that would always be a pig to rule - at least unless your genocidal - and you would gain very little from it.
The big problem of course with a big [and presumably stronger] empire is that Acton's Law comes into play. Britain got too complacent in its position as it was. Make it so powerful that it need fear no opponents and potentially you have a very dark situation.
Steve