Hi Darren,
Im bias in so many ways but thought id say hi, noticed its your first post, and what a great site and im a regular reader.
My initial thoughts are 2x banner blocks wouldn't really generate revenue. Banner space is what companies looking for branding or a presence look for and hence why they pay an up front fee. With your traffic though you might get a few sales.
With the content your site has you could easily work in some affiliate links and generate a nice income without feeling like you've sold out

For example you mention review sites quite a lot, Holiday Watchdog has an affiliate programme via Affiliate Future that you could replace the standard link with an affiliate one. This is the type of approach Money Saving Expert takes and they flag affiliate links with a * after the anchor text to let their users know.
Your still giving the same service but making some money for yourself as well.
Once you get into it you'll realise a lot of the companies you mention on your blog actually have affiliate programmes or there are companies that could compliment your content.
And for your q's:
Do companies offer this size ads?
Off the top of my head I dont know, but id imagine any would be more than happy to knock you one up
Which are the best affiliate networks?
Its personal preference. Have a look on them all and get to know them, its free sign ups on all (bar Affiliate Window) so theres no risk to get to know how they work, how to get links, what merchants they have etc.
Does anyone have any tips to get me going?
As above really. You've got traffic in the right vertical, sell in context and you'll be seeing sales in no time
What do you think Google's opinion is of affiliate advertising?
Depends on the site. Some it hates, some it loves