The tank size is not that important, its the delivery in terms of cfm (cu ft per min). If the budget allows I'd go for a 3hp and a smaller tank. It's always better to have a big machine doing a smaller job than vice-versa.
For example if your spraygun needs 12 cfm and the compressor will only supply 10 you will keep having to stop to let the compressor catch up, but if the compressor supplies 14cfm it will keep on top of the job.
A big tank is useful in a garage where you may be using air tools occasionally, it will fill up, you'll get quite a lot of use before the compressor kicks in again, that way the compressor isn't continually switching on and off.
If you find the tank isn't big enough you can always add another receiver at a later date, I've got 150 litres + 3 x 20 litres worth of calor gas bottles fed with a 3hp compressor and 9 fridge motors for my grit blasting operation, but that's a different story!
Watch out for Machine mart's misleading CFM figures. I believe they quote what the compressor draws in, NOT what it will supply to free air. Big difference when it comes to running spray guns or other tools you run for a reasonable period of time.
Watch out for the words 'free air delivery', in cubic feet per min, this what you should compare to the indented tool's air consumption.
Oh Alex, will you get off that Aldi business! You own a Portamig mate, thought you demanded quality?!
If I see bargains I tell the boys... Helping each other is what it`s about....I think.........