Hi Bill and welcome to the forum. Any bit about 1 1/2" or over in diameter is likely to be 1/2" shank. Anything that large would apply too much torque on a 1/4" shank. I have seen a few vertical raised panel bits in 1/4" shank but I wouldn't recommend them. You would need to make a bunch of light cuts to get to the finished profile. If you want to make raised panel doors I would do what James suggested.
You didn't mention which router you have. My Bosch (assembled in Mexico) 1617 has both quarter and half inch collets that are interchangable. Check on the web for a half inch collet for your brand of router. If you can't get a half inch for your router, I'd follow the advice of getting a router that has one. I agree that a quarter inch shank is probably too small for a large bit, and if your router is underpowered, you will have to make many light passes at very slow feed rate or risk. By getting a stronger new or used router with half inch collet you'll have a safer, more successful experience. By the way, welcome to the forum, many very nice and helpful people here with LOTS of experience.
Welcome to the forum Bill.
I would also suggest that you just get a router with a 1/2 in. collet and be safe. A big router bit to do raised panels will need at least a 2 & 1/2 hp motor to do the job. You never have too many routers and more power the better. You will want a "Soft Start" motor as well, so don't go cheap.
You can also easily make raised panels on a table saw. You might need to make a few adjustments as you go, but the job IS safely done without "effort overload"
There are lots of instructional clips on "youtube" about making raised panels on your table saw.
Wishing you great results and good sawdust!! Let us know how you manage, please.
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