You may want to read some of the excellent guides available on this site but here are MY answers:
1) it can, especially at first or if there is knotting involved
2) you can feel it swelling inside you. You can sometimes get the feeling that you are being inflated. In actuality, a dog’s cock will start to swell immediately upon commencement of penetration but that swelling accelerates upon knotting. The relatively large volume of seminal fluid being delivered can add to this feeling of being filled, especially if the knot is properly in place to block it from seeping out.
3) stretch your ass gently over time to accustom yourself to the introduction of a dogs penis and/or his knot. Consider voiding your bowels and possibly using an enema just before sex to clear out any unwanted material from getting in the way.
4) generally, the bigger the penis, the more stretching and feeling of fullness will be involved. This said, the bigger the penis, the more chance of their being pain involved in the act of accommodating its insertion.
5) there is such a thing as too big. Know your limits. They are not set in stone and with a little training and practice you’ll be able to increase the length and girth that you can comfortably accommodate but it is foolish to try and take too big a cock too soon, especially because dog sex can be very forceful and abrupt. You don’t want to tear the walls of your rectum or get stabbed too hard in the scrotum or the inner lining of your bowels.