Hi . I made a crocheted one that stretches over the end of our bed . For the width a just stretched out my arms after chaining and just thought 0k about 260 stitches . Using a dk yarn and dc and 1 chain ( crochet linen stitch ) it was 6.5’ long in the end . And that I think will be your issue if you want to make a lap blanket with lines to get the full effect of the temperature change. It wouldn’t be much different with knitting .
However if you really want a lap blanket I suppose you could do granny squares or mitred squares . But I don’t think the effect would be the same . There are plenty of other designs out there on Ravelry for example . I quite fancy doing a minimum and maximum one one day , with a swirl created down the middle by altering the daily stitches by 1 stitch changing from the min to max colour.
As far as data goes , I used the weather forecast daily because I was doing it live , wherever I was on that day . I generally made it daily rather than catching up. Recorded temps one week when it was getting huge and I didn’t want to take it with me whilst away !
if you are using historic temps or just want to think about colour selections then the website temperature-blanket.com is brilliant. Not only are there historical temps for anywhere , you can input your chosen colours for any place at any date and look at why would be created .
Yarn choice is also a whole other area to think about . I would recommend using 50g balls rather than 100g balls. Just because you may end up with lots if unused yarn - unless that doesn’t bother you ! Go for a brand with a good colour range . I wanted to use wool so ended up using drops fine merino which is a dk. Not too expensive too!
You need to think about colour range too and this is where that website comes in useful too. I used 1 colour for 2 degrees but one for anything over 30 c and under 0 c . But that really depends on the climate where you live. I decided on my range and made a chart with a sample of the yarn tied through a punched hole in a card with the temps written beside it . I took a photo of that on my phone too so that I could refer to that sometimes without needing the card with me.
The other thing to remember too is that you don’t need to buy all your yarn at once . If you start in winter just buy the cooler colours and move onto the warmer ones as it gets to spring and summer .
Lastly when I finished I bought a large wooden button on Etsy with the date burnished on it to record my work.
i am sure others will be able to give you other tips
Good luck!