are these t shirts tight?

From the looks of it they're designed to be somewhat close fitting but not tight. It would depend on your build. Could you order your normal size and then the next size up to check fit, keep whichever you prefer, and return the other?

thats a good idea but am not sure can you return them right there and then when u try them on

Usually online retailers will include a return form and return label in the parcel. So you'll just fill in the form (usually it will give you the option to tick a box that explains why you're returning it and whether you want a refund or replacement) and then you just pack whatever you're retuning back in the original packaging along with the return form, and with the new return label stuck over your address. Then you can just take it to the post office (or other package delivery service) and send it off.
Obviously if you're buying something in person you can just try different options on in the changing room (if provided) before buying.