EasyJet - what size is 'under Seat bag' ??

No more than :

8.2 x 13.2 x 16 inches
or
45x36x20cm

Unless you have paid more for flexi fares etc or are an EasyJet Plus cardholder, you only get one piece of hand luggage, with a maximum size of 56 x 45 x 25. Any handbag and laptop will have to fit into this bag.

If you have paid a premium, you can also take an extra piece of hand luggage, measuring 45 x 36 x 20cms, which is 17.7 x 17.2 x 7.8 inches, not the imperial measurements stated in the first post.

EasyJet’s website has a very straightforward explanation if you Google this if you have further queries.

https://www.easyjet.com/en/help/baggage/cabin-bag-and-hold-luggage

Jennifer's conversion doesn't look right to me. 36 cm. is considerably less than 17.2 in. It's just under 14.2 inches, so I'd guess "17.2" was a typo.

It's really in centimeters, as stated above, but translates roughly to inches. It's about half the size of a regular carry on bag.

Soft sided is a good idea. I hate to think of the problems if there is some obstacle that keeps your bag from fitting?

I flew Ryan Air in May (Dublin to Leeds and back). I was a little concerned whether the space would be smaller for a window seat. Fortunately my bag fit.

The position of the seat-mounting brackets often seems to reduce the space available for storing bags in front of window and aisle seats and allow extra space for middle-seat bags.