Should I worry about a fund's size or whether it might close?

For mainstream index funds, no — they're enormous, stable, and unlikely to close. Fund size matters more for small, niche, or new funds: a fund with very little money under management can be shut down or merged by the company if it stays unprofitable, which forces an unplanned sale that may trigger taxes in a taxable account and leaves you reinvesting elsewhere. Very large funds in narrow strategies can face the opposite issue, where their own size makes it harder to trade efficiently, though this rarely affects broad index funds. Practical guidance: favor established, sizable funds from major providers for your core holdings, and be a little cautious with tiny or brand-new specialty funds. A multi-billion-dollar total-market index fund isn't going anywhere. You can check a fund's total assets on its fact sheet before investing.