Go to
app/design/frontend/[your_package]/[your_theme]/template/checkout/cart/item/default.phtml
and find resize(75) and change the width and height
<td><?php if ($this->hasProductUrl()):?><a href=”<?php echo $this->getProductUrl() ?>” title=”<?php echo $this->escapeHtml($this->getProductName()) ?>” class=”product-image”><?php endif;?><img src=”<?php echo $this->getProductThumbnail()->resize(75); ?>” width=”75″ height=”75″ alt=”<?php echo $this->escapeHtml($this->getProductName()) ?>” /><?php if ($this->hasProductUrl()):?></a><?php endif;?></td>