Speaking as a petite woman (5"1, 110lbs) who has been training for 7 years now, I would have to second Lynn Seiser's response in regards to asking your training partner instead of simply making an assumption about how capable they are. I could tell some funny stories about guys on the mats making assumptions about me due to my size, but that's for another time.
Seriously though, I can certainly appreciate that in asking your question, you're wanting to be considerate of the other person.
The other thing to keep in mind is that if this person is new to Aikido, then like all other new students (regardless of gender) chances are your instructor would prefer that beginners refrain from challenging each other with excessive strength right off the bat so as to give each other "room" to learn proper basic form first. If it seems like your training partner is struggling, you can always ask them if they you to ease up or slow down a bit so that they can concentrate more on the form - and then leave it up to them to decide how they wish to practice.
While I personally enjoy having to move training partners much larger than me, I can also appreciate that when you're a complete beginner, you're already pretty occupied just trying to learn the specifics of how to move and that one of the last things you need (regardless of size or gender) is to think about how to adapt the basic form to your partner's extra force. You're training, not fighting, and the intent is to help each other learn - so definitely when in doubt: ask, don't assume.