The majority of women's vaginas are not too small for sexual intercourse. The vagina is designed for sex and also to give birth to babies. The vagina does change a lot over your lifetime, in terms of shape and elasticity. When you are sexually aroused, the vagina will usually elongate. A natural lubricant is also produced which makes penetration easier.
Having said that, up to one-third of women do report discomfort during intercourse. This may be because of inadequate lubrication, for example if penetration starts too soon, before there is enough arousal to produce lubricant, or a condition called vaginismus, which is an involuntary tightening of the pelvic floor muscles. Infection may also cause pain, and there are some congenital conditions that can also lead to difficulties.
The key thing is to make sure you are sufficiently aroused and lubricated before attempting penetration, rather than rushing things. If you are concerned there may be a physical issue, being examined by a gynaecologist can help to find out if this is the case or not.
Hope this helps!