5'7 you'll be fine. The younger generation is actually kind of taller since nutrition isn't a huge issue as it was before. They'll still frail. No one lifts.
The 90s were a completely different era for Cambodia. It has changed so much then.
But your dad is right. More people tend to get mugged at ATMs. You don't seem like the type that'll go party until 2-3AM in the morning in a shady part of town, but I almost got robbed in a tuk-tuk before I pulled my phone out to GPS my way back to my hotel.
I traveled light and bought most of my souvenirs and clothes towards the end of my trip because I didn't want to carry around a lot of stuff for most of my trip. I have had a Poler backpack. There is only one pocket in the front that I kept a razor/toothbrush in. Super sturdy. I packed light. I wore 1 pair of jeans throughout my entire trip and maybe 3-4 shirts. You can do laundry in the market if your relatives don't have washing machines.
Last advice: Eat lots of fruit. Please take advantage of the local street fruit vendors It's like 25 cents for a fully cut pineapple. I wish I ate more food when I was there.
If you care to read this is how Cambodia affected me:
Cambodia changed me completely even after months after leaving. Even though I was connected with my cultural roots here in the U.S. with the local community, I was still American. Results oriented, bigger is better, become more rich and add your 'wealth' constantly, which is all good, but you realize how much a rat race American society is. I was always raised to try to be educated, touch, respectful, 'high class' and try to be best at whatever I did so the other Cambodian guys who were hentai, B-boying, video game obsessed losers or gang-bangers who I thought didn't do too much was all I had known. Only people who called our house at 3AM in the morning were Khmer people who wanted to solicit my dad for a MoneyGram. I had such bad connotations of my own people. I thought my own grandma was manipulative as heck. When I went there, I went from sleeping at my aunt's house who made less than $5 a day to dining at $25/dollar a plate (which is SO FUCKING MUCH in Cambodia) restaurant with the largest beer distributor in Battambang. I saw how inequality and capitalism isn't something uniquely American. My cousin who is 4 years younger than me can fix phones fast and better than I can despite only doing it for a $1-2 profit every time while I'm making $25.
Khmer people are friendly, holistically good, extremely generous, hospitable, hard-working and so naturally curious about everything. We were cleaning up our grandma's house (she passed) and her neighbors all help come by to help clean up the overgrown yard. Sure they gossip, but in the end most Khmer people do the right thing (if you're alive). We crashed our car driving Sihanouk and an entire village gathered around and helped us, but if we had died and gone into the stream we were headed into entirely, no doubt our luggages and bodies would be looted. But other than that there was no sense of malevolence or foul play there. No anxiety, no insecurities or bullshit that you may experience here in America. It was all created in our minds. The happiest times is where I would catch someone blankly staring at me and I would respond with a smile. The first and natural response for them to do is smile back. I'm obviously a bit cynical by nature. I felt at home though, accepted, and that ethnic homogeny that being in racially diverse and aware America deprives you of your whole life enveloped my whole experience. It's true Khmer culture. It's in our blood. So whenever I feel down or lazy or negative about something I am doing in my life, I just think back to my time in Cambodia and how there is a life that is greater than this Western routine and that the world is much bigger than me and whatever is going in my head. I may be a bit sensational and your experience will differ from mine. You can say I kind of grew up.