Not to sure if the EU should be made larger.
First, the problem with more nations being in it, is that there are more voices, the need more more agreement, and therefore less being done. An example of there consensus is harmful is the EU common agricultural policy. It can not be changed as too many people are against such change. Get more people on board, and it will be even harder to agree on issues.
Second, is immigration. Like it or not, many Polish people and others from Eastern Europe have come to the UK to work. And they do work, and work damm hard. The problem is that it has bred resentment. A shame as there are too many people in the UK who could work, but are psychologically dependent upon such benefits, wanting a free ride. (Please, note, I do not think that everyone on benefits is after a free ride.) Locally trained builders for example are being undercut by Pole's who have less overheads and living costs, therefore putting them out of business, through no fault of their own. While the influx of such labour has benfitted the UK, it is also causing resentment.
Third is jobs. If other such countries are admitted to the EU, my fear is for UK jobs. Labour costs are less in Eastern Europe, which means that it will be financially advantageous for such companies to relocate, especially as they will not need to pay trade tariffs.
Lastly, I work in health care. I have concerns about some doctors from abroad. I have worked with excellent doctors from Germany and Spain (and have yet to meet a bad one) as well as a brilliant doctor from Italy. But I have met someone from Romania, who thanks to EU enlargment, would be able to work here. And sorry, but that doctor is crap and dangerous. Thanks to EU enlargment, we are getting loads of doctors from Eastern Europe, who have poor English skills, and now are having to turn away skilled doctors from India and Africa who have excellent English (well, compare them to those from Eastern Europe and say that) and due to the exams they have to sit to work in the UK, have to meet a standard of medicine that those from Eastern Europe do not have to meet. I am aware however, that there will be excellent doctors from Eastern Europe, I just have yet to meet one.
Saying that, I am in favour of the EU. But at the moment, it needs reform. It wastes too much money and makes too many bad decisions. It can do so much more for the people of Europe, and we, the population of the EU, deserve that.