Sizing

I thought it would be best to start with some sizing advice.  I tend to use double knit (DK) yarn as this seems to be more readily available in the UK as there is often a section of craft items in most chain bargain stores.  Obviously, there are many places online where you can buy your yarn but just be careful when doing this that you actually by the right type of yarn that you need and watch out for the weight on the balls too! 

Knitting

Using DK yarn, the average inch is 5.5 stitches.  So, for example if you want to make a purse/wallet that was 6 inches wide then you would need to cast on 33 stitches (5.5 x 6).  If the number works out with a half or quarter stitch, I usually round this up.

Generally, using DK yarn means using 4mm needles however, you can use up 4.5mm needles if you prefer a looser stitch.  I find that using 4.5mm needles for a hat for example gives a little bit more room and isn't as tight on the head - but this is personal preference.

In terms of the length of the project you can either use a row counter or simply measure the length with a tape measure until you reach the required amount.

Crocheting

I find sizing in crocheting much easier than knitting as I tend to use measurements not a stitch count.  I simply use a tape measure and measure the foundation chain.  If the foundation chain is 'right on' the size, I always add my turning chains to make sure I keep the right size. 

Hook size - most yarn manufacturers tend to size down the hook size needed for any weight yarn but the hook size is entirely up to you.  3mm - 4.5mm hooks all work great with DK yarn, obviously the bigger hook you use, the looser the stitches are going to be.  

As with knitting, in terms of the length of the project you can either use a row counter or simply measure the length with a tape measure until you reach the required amount.

Side note: I use a tension ring when crocheting as I find this easier to hold the yarn