Econ at SOAS is good - we were ranked 4th in the country for 4 years straight, though we dropped a little this year because of the inclusion of NSS to the rankings - I wouldn't worry too much, SOAS students have a habit of complaining about everything.
First year lectures are pretty big 'coz they're often shared with develoment studies students.
Classes get smaller through the year as attendance drops, and numbers will fall over the next 2 years as the classes get more specialised. Tutorials will usually have a max of 15 people in them; by my 3rd year some tutorials only had 3 people in them.
The actual choice of modules for Economics and Dev. Econ. is exactly the same, the only difference is that the quantitave and pure economics modules are compulsory/core for Econ, whereas Dev. Econ.'s compulsory courses are focused on development.
There were very few Dev. Econ. students in my year - the number increased a little going into 3rd year because Econ. students that get bad grades in core courses either have to repeat, or drop down to Dev. Econ.
Research for most modules is based on a variety of readings, the main ones are included in study packs, the rest are available online; only the Econ. Analysis really uses a textbook, the Quantitative Techniques class is easy for anyone with A level or IB math, and the teaching is straightforward so I wouldn't get a textbook.
The only book I would buy in my first year is the one for Economic Analysis by Begg. Later on it's often convenient to buy books 'coz there's no way the library can cater for 100 people that want the same book, though you can borrow from the Senate House library and LSE library too.
The first year is very much focused on being an introductory experience, you only learn the basics, and you get to do a floater.
Personally, if I started off in Dev Econ and got good grades in Quantitative Methods and Analysis I would get my degree changed to BSc Economics (which is what I did (as advised by the 2nd year supervisor)).