Depending on your exact size, there's a good chance that bike shops won't have anything in your size, anyway. Bicycle-specific and athletic type clothing can be had in larger sizes, but, sadly, it will be almost all mail order and you'll have to do a lot trying on and shipping back and forth. It's the curse of larger people.
You can get cycling-specific bottoms (with pad) in 3/4 length and full-length. You can also buy compression "sleeves" that go from just above the ankle to mid-thigh that you would wear with regular bike shorts.
I wear Under Armour Heat Gear compression bottoms over bike shorts for cooler weather (up to about 55 F),and I almost always wear a pair of lightweight nylon shorts over bike shorts for a more "conventional" look. And a plain synthetic t-shirt...I don't really feel like I need cycling jerseys.
A lot of this is going to depend on how comfortable you are with your body (i.e. how close-fitting the clothes are and how much skin showing), but the one thing I think it's important to emphasize is that you wear some sort of sport- or performance-oriented synthetic fabric. There are long-sleeved and long-leg options designed for warmer weather. Jeans/sweatpants/hoodies will definitely trap way too much heat and moisture and make you miserable.
You don't mention your gender, but Aerotech Designs in the US sells Men's sizes up to something like 5XL and 4XLT, I think. For reference, I'm 6'2" and 295# and I find the size L bike shorts at Aerotech to be the right fit for me, even though the size chart points to an XL based on measurements. Not sure about women's sizing. I've heard of a UK shop, "Fat Lad" or something like that, but I haven't done any business with them.
In short (pardon the pun) I think cycling-specific bottoms is probably a good investment, anything else is more about fit, fabric and personal comfort and doesn't need to be "bicycle clothing".
Last edited by MiPeGr; 03-09-20 at 12:40 PM.