But how many rows?!

So, I just found out I’m going to be a grandma! Obviously I need to start knitting immediately. They showed me the ultrasound and asked what we should call him/her: Peanut or Bean - I said Bean and instantly knew I would make a blanket to tell Bean ‘hi’. I want to make a baby blanket in stockinette with a seed border. My struggle is I can’t find # of rows to knit and I need to know so I can find the center to add ‘Hi Bean’ in a different color. I tried some math but a 151 co and 144 rows is just too narrow in my graphing. I’ve got #4 Feels like Butta yarn. I’m thinking around a 34x40in blanket? Bigger than a stroller blanket, crib-ish size (my heart sees this child carrying this blanky everywhere knowing Grandma says hi). I’ve searched online and can’t find any pattern that gives total rows, just 'repeat until xx inches '. Lots of guides for cast on though. Any suggestions? (I’ll add the letters in the same pattern, just a different color, with Hi above Bean)

Congratulations! How exciting!

Not everyone likes to swatch, especially when excited to get started, but swatching really would be helpful to find out how many rows and where the centre is.
If you cast on with your selected needles and yarn, enough stitches to make 20 cm and knit as many rows as it takes to make a square. Then wash or block following the laundry instructions on the ball band and let it dry. You can then get a reasonable calculation from the centre 10 by 10 cm square ignoring the curled edges and from there can calculate the number of rows needed to get whatever measurement you want for the blanket. You can also calculate how many rows before the text and how many rows the text will take.

The swatch can even become a separate, smaller lovey for easy carrying, a border could be added later if you fancied this.

The best reason to knit! Congratulations.
You might want to chart your letters on knitter’s graph paper to make sure they look the way you want. This may not be necessry depending on your gauge and how large the letters will be.
https://www.sweaterscapes.com/land-chart-paper.htm

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You can also measure the length of the kntting to estimate when you are close to the center of the blanket.
I love the idea of the hello to Bean.

That looks like a great link for graph paper, I’ve saved it, thanks.
ETA and some great picture kntting patterns too!